Does the error log show any clues? That's usually where I go when seemingly random crashes show up. Also, if you haven't already, it might be a good idea to switch off the automatic rebooting on system errors. If you let the system go ahead and show the blue screen instead of restarting, it can reveal the cause of some stop errors.
The last time I started getting random crashes like that (and it's been a long while, knock on wood), it was a RAM stick going belly-up.