I agree with Curlyben -- especially in light of your previous problem, this is probably a hardware issue.
If you have eliminate overheating (which is the most likely cause, and something you should investigate thoroughly), you'll want to use the process of elimination to determine what hardware is causing the problem.
Disconnect any non-essential hardware.
Try repacing hardware with any spare parts you have or can borrow. Don't forget the PSU.
There is a software memory tester available at
http://www.memtest86.com/memtest86-3.2.iso.zip
Unzip and burn the image to a CD and boot from it. Let the test run for about 5 minutes, if you see errors, your memory is (probably) bad.
;)
psi42