How do I diagnose the source of crash on my MacBook Pro? My Intel Core Duo MacBook Pro 15" has been crashing more and more.
Previously there were various types of screen-freezes.
Now the screen-freeze is being followed by an auto-reboot, which suggests to me a serious memory fault. Auto-reboot is a 3-5 second freeze followed by sudden system reboot, that reloads the OS like normal.
I have backed everything up and run Drive Genius programs to defrag and verify the drive, but the problems have come back.
My current plan is to replace the hard drive.
My question is: what else could be going on here?
Can RAM or CPU chips be the of cause this? Is there any way to check that?

Screen-freezes include any of the following:


everything but the mouse freezes
everything freezes but no graphics distortion
all graphics distortion (whole screen; couple times with trippy mouse tracer effect)
all graphics fade to an eerie pale blue color (3 second fade time)


Specs


Processor 2.16 Ghz Intel Core Duo
Memory 2 GB 667 Mhz DDR2 SDRAM
100GB hard drive (profiler doesn't show me the model) 
This is the first generation of Intel MBPs; purchased new in 2006 I think.
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.