Latest update on lockups

Alright, heres the latest on my lockup issues. I've got my KT400 down to 1 stick of Mushkin 256mb DDR333 with a heatspreader(lowest timing settings.) I've got pretty much all the updated drivers for everything. I've installed a thermalright slk800 heatsink with AS3. I keep getting lockups during games. If I leave a house fan next to the case blowing in, this pretty much resolves the issue. So that brings us to: heat. Could the video card be giving me fits? (even though it came stock with a fan and heatsink on it.) Or could it be the northbridge getting too hot?

Any ideas are appreciated.

Brian