Originally posted here by jenjen
at first it sounds like you have a memory problem but you say you've tried that..

just for grins and giggles try running docmemory and memtest86

docmemory
http://www.utilitygeek.com/details.php?fileid=13
memtest86
http://www.memtest86.com/

now here's a thought/wild guess.. there are/were a bad batch of capacitors used on many motherboards over the past few years.. inspect all your caps for bulges or leaking.. most are located by the voltage regulators and there's usually one by your apg slot..

here's a good site run by a guy by the name of topcat, who specializes in replacing caps on bad mobo's.. http://www.badcaps.net you'll find a lot of good info there..
Memtest is good and has helped me figure out RAM troubles...