I'm currently running freebsd on my desktop (core i7 920, 6gb of ram). Since around January I'm having problems with the system freezing for short periods of time and eventually locking up due to heavy reads/writes to the disk. 
The cursor would freeze for a split second and songs in my music player (quodlibet) would skip completely (the error logs say that the music player was unable to access the song which was currently playing, even though my music collection is on a completely different hard drive than the os). 
After 10-20 minutes of the occasional freezes (the frequency is random, sometimes it can go without any freeze for some minutes or it can freeze many times in a short period of time) the system locks up completely and reboots itself. There is no crash dump produced by the kernel, nor anything shows in dmesg when the short freezes occur. 
I tried putting freebsd on a different hard drive, changing filesystems from zfs which I currently use to ufs but the issue doesn't go away. 
Today I tried using the geom rr sched which was recently committed to current to see whether it would make a difference, but it didn't help.
This issue is not related to swapping as ram usage never gets high enough, and is also not directly caused by heavy cpu usage, although it might be part of the problem.
I've done several clean installs of freebsd current since the issue first appeared.

How-To-Repeat: make -j8 buildworld would trigger it in my case, or 2 simultaneous while loops with extracting a large file archive or compiling something big like openoffice/gcc/boost from ports.