Freezes on heavy disk I/O

Hi!

I've got a box running FreeBSD 5.2.1. Sometimes, it just
freezes. Doesn't react to _any_thing except a hard reset.

This is reproductable in some cases, all involving mainly
disk I/O.

1) When burning a CD (on another computer) from a samba share
on the freebsd box, the box repliably freezes. This also
happens when I'm encoding a DVD rip to avi on my windows box,
while the DVD files are on the freebsd samba server.

2) When doing stuff like portupgrade -a, it often (but always
at different files/ports) freezes. This happens especially
often when compiling gcc. (That's why I'm using the package now).

3) Sometimes it just freezes like this, when playing an mp3,
surfing the 'net, etc.

I used memtest enabling all test for 9 hours finding no errors.
Also, it's no overheating problem as it is not overclocked
nor is there any hot air coming out of the CPU fan when those
freezes occur.

Any ideas?

Kosta