There might be some info in /var/log/messages or /var/log/syslog - it depends on how the crash is happening. If it happens regularly you could try running top in a terminal window and keep an eye on what is happening. The sysstat
utilities might let you see if there are high loads on your system (CPU, RAM, IO, etc.) prior to the crash.
Also, are you getting hardware warnings for anything (temperature, etc.) in your BIOS? Have you run something like memtest from the system rescue cd
to check your memory?