panic occur at intervals ranging from several hours to several weeks.

A similar problem: kern/137881,kern/152893

environment:
CPU: Intel(R) Core(TM)2 Quad CPU    Q8400  @ 2.66GHz (2660.01-MHz 686-class CPU)
real memory  = 2147483648 (2048 MB)
avail memory = 2071199744 (1975 MB)
ACPI APIC Table: <INTEL DG43NB>
acpi0: <INTEL DP43TF> on motherboard
igb0: <Intel(R) PRO/1000 Network Connection version - 2.0.7>
igb1: <Intel(R) PRO/1000 Network Connection version - 2.0.7>
em0: <Intel(R) PRO/1000 Network Connection 7.1.8>
em0: Using an MSI interrupt
em0: [FILTER]
                                                                                                        
loader.conf:
hw.em.rxd="4096"                                                                                                     
hw.em.txd="4096"                                                                                                     
hw.igb.rxd="4096"                                                                                                    
hw.igb.txd="4096"                                                                                                    
hw.igb.lro=0                                                                                                         
hw.igb.fc_setting=0                                                                                                  
hw.em.rx_abs_int_delay="33"                                                                                          
hw.em.rx_process_limit="-1"                                                                                          
hw.em.enable_msix="1"                                                                                                
hw.em.msix_queues="2"                                                                                                
kern.ipc.nmbclusters="128000"                                                                                        
net.graph.maxdata="1024"                                                                                             
net.graph.maxalloc="16384"                                                                                           
net.isr.maxthreads=2                                                                                                 
net.isr.numthreads=2                                                                                                 


sysctl:

net.graph.recvspace=256000                                                                                           
net.graph.maxdgram=128000                                                                                            
net.route.netisr_maxqlen=512                                                                                         
net.inet.icmp.icmplim=2000                                                                                           
net.inet.ip.fastforwarding=1                                                                                         
net.inet.tcp.blackhole=2                                                                                             
net.inet.udp.blackhole=1                                                                                             
kern.ipc.somaxconn=1024                                                                                              
net.inet.ip.portrange.first=1024                                                                                     
net.inet.ip.portrange.last=65535                                                                                     
net.inet.ip.portrange.randomized=0                                                                                   
net.inet.tcp.nolocaltimewait=1                                                                                       
net.graph.maxdata=1024                                                                                               
kern.ipc.maxsockbuf=524288                                                                                           


Fatal trap 12: page fault while in kernel mode                        
cpuid = 0; apic id = 00                                               
fault virtual address   = 0xa1aabda0                                  
fault code              = supervisor write, page not present          
instruction pointer     = 0x20:0xc08c464c                             
stack pointer           = 0x28:0xe7f71918                             
frame pointer           = 0x28:0xe7f7193c                             
code segment            = base rx0, limit 0xfffff, type 0x1b          
                        = DPL 0, pres 1, def32 1, gran 1              
processor eflags        = resume, IOPL = 0                            
current process         = 3983 (mpd5)                                 
trap number             = 12                                          
panic: page fault                                                     
cpuid = 0                                                             
KDB: stack backtrace:                                                 
#0 0xc08e0d67 at kdb_backtrace+0x47                                   
#1 0xc08b1e17 at panic+0x117                                          
#2 0xc0be1af3 at trap_fatal+0x323                                     
#3 0xc0be1fd0 at trap+0x180                                           
#4 0xc0bc8eec at calltrap+0x6                                         
#5 0xc08c4759 at _callout_stop_safe+0x89                              
#6 0xc7c37bef at ng_uncallout+0x1f                                    
#7 0xc7d8f0a4 at ng_pppoe_disconnect+0x74                             
#8 0xc7c38f58 at ng_destroy_hook+0xe8                                 
#9 0xc7c3a4a5 at ng_apply_item+0xf35                                  
#10 0xc7c3874f at ng_snd_item+0x28f                                   
#11 0xc7c32642 at ngc_send+0x1c2                                      
#12 0xc0913f6a at sosend_generic+0x64a                                
#13 0xc091003f at sosend+0x3f                                         
#14 0xc0917931 at kern_sendit+0x161                                   
#15 0xc0917b61 at sendit+0xb1                                         
#16 0xc0917c78 at sendto+0x48                                         
#17 0xc08eca99 at syscallenter+0x329                                  
Uptime: 3d3h21m46s                                                    
kernel trap 12 with interrupts disabled                               
                                                                      
                                                                      
Fatal trap 12: page fault while in kernel mode                        
cpuid = 0; apic id = 00                                               
fault virtual address   = 0x0                                         
fault code              = supervisor write, page not present          
instruction pointer     = 0x20:0xc0bdf9ce                             
stack pointer           = 0x28:0xe7f71758                             
frame pointer           = 0x28:0xe7f7177c                             
code segment            = base rx0, limit 0xfffff, type 0x1b          
                        = DPL 0, pres 1, def32 1, gran 1              
processor eflags        = resume, IOPL = 0                            
current process         = 3983 (mpd5)                                 
trap number             = 12                                          
panic: page fault                                                     
cpuid = 0                                                             
Uptime: 3d3h21m46s