strange server reboots

[Wojciech Puchar]

every 2-3 weeks i got server reboot. no panic no errors just reboot.
server are connected to UPS it's not power problem, and ALWAYS the same 
after effect is comming - squid's swap.state files are wrong (possibly 
emply), squid "sees" empty cache after that reboot.

i have to rm swap.state* and rerun squid and wait quite a bit time until 
it rebuild database.

any idea?

FreeBSD 6.2-RC1/amd64