How to troubleshoot why VirtualBox kernel module freezes the system?

[Yuri]

VirtualBox kernel module (port emulators/virtualbox-ose-kmod) began to 
cause system freezes from some kernel change around the end of January.
Once the system freezes it has to be rebooted. Soundcard, if it was 
playing something, begins to cycle some very short piece.
Nothing is logged into /var/log/messages.

Is there any way to troubleshoot this, like enabling some kernel 
configuation options?
What is normally done in such case?
Should I run system under kernel debugger? But would it detect something 
if it's not a SEGV but the freeze?

Yuri