|It sounds like the bottom of the heat sink is not flat on the cpu.|
It can be very difficult to see whether the heat sink is flat on the cpu when you do that when you have the mboard in the case - you can only tell for sure by using a small mirror and good lighting.
"Product: Amd athlon / 2000+ thoroughbred"
The proper heat sink for your cpu has a notch on the bottom that must be above the raised side of the socket A socket - there is only one way it can be properly oriented.
This is unlikely, but it CAN be, especially for an older mboard like yours......
E.g. This can cause all sorts of problems, including black screening and re-booting for no apparent reason.
examine the mboard to see if you have bad capacitors, and/or other findable signs of mboard damage .
This was the original bad capacitor problem - has some example pictures.
History of why the exploding capacitors and which mboard makers were affected:
What to look for, mboard symptoms, example pictures:
Home page that site
- what the problem is caused by
- he says there are STILL bad capacitors on more recent mboards.
Pictures of blown capacitors, other components, power supplies, Athlon cpu's, etc.: