Time to go fishing in the motherboard manual again... and perhaps the AMD website.
What you're looking for is the correct CPU voltage for your processor. The motherboard manual might not have that information if you had to upgrade the BIOS to get it to see your processor correctly. But it will have a jumper setting you can change to get the voltage right! This is sounding like *the* core problem, BTW.
Argh... Asus now have Opera-hostile JavaScript on their website. And I couldn't find out from AMD's site what voltage an Athlon 2000 is supposed to run at. :-( If no other information can be found, drop it in 0.05 increments and see what happens.
Wade.