How to diagnose system freezes?

[Yuri]

One of my 9.1-BETA1 systems periodically freezes. If sound was playing, 
it would usually cycle with a very short period. And system stops being 
sensitive to keyboard/mouse. Also ping of this system doesn't get a 
response.
I would normally think that this is the faulty memory. But memory was 
recently replaced and tested with memtest+ for hours both before and 
after freezes and it passes all tests.
One out of the ordinary thing that is running on this system is nvidia 
driver. But the freezes happen even when there is no graphics activity.
Another out of the ordinary thing is that the kernel is built for 
DTrace. But DTrace was never used in the sessions that had a freeze.

What is the way to diagnose this problem?

CPU: i7 CPU         920  @ 2.67GHz
Memory: 24GB
MB: P2T

Yuri