Re: apparent crashes persist.
vesa drivers without nvidia glx: firefox still crashes, only more
slowly. Look like nvidia's stuff is off the hook.
I'd definitely try memtest86+. Install, reboot, pick the option from
GRUB and let it run overnight.
I have a similar setup to yours, and was having random lock-ups, etc.
Turns out I had some dodgy RAM that memtest86+ picked up within 10
minutes of running. Replaced with a proper brand (GEiL); works a charm now.