How do I diagnose the source of crash on my MacBook Pro?
Doesn't sound like a hard drive failure to me; maybe RAM or - worse - anything logic board.

Try the already mentioned AHT and try to find error logs via Console (if you don't know where to start looking, try narrowing your options by looking at the timestamps, they'll pretty much stop after whatever went wrong and then continue after the reboot with a noticeable time shift)

Other things you could try:


Verifying Volume and File Permissions, ideally while booting from a
recovery/install disc, not the installation on the hard drive.While
the graphical errors don't seem to be caused by a hard disk problem,
the OS crashes themselves might be because of corrupted files that
cause problems when being read/written.
Running a memory test and / or removing RAM modules to see whether one of those is faulty. In my experience though, faulty memory is
more likely to result in the device not starting up at all.
Try to check the temperatures of various parts of your machine, e.g. with iStats (and/or check whether you can simply feel your
machine getting noticeably hot). Graphical errors and/or freezes
might be due to overheating of components resulting in an emergency
shutdown to prevent damage to the hardware.