Why are you running two types of RAM? PC2700 (or DDR 333) and PC3200 (or DDR 400) have different RAM timings in relation to the front side bus. Can you tell us what speed your FSB is running at? It might be that you've pushed the stick of PC2700 RAM too hard because it really shouldn't operate at 200MHz.

Anyway, I don't know if that could be the problem. Could be heat issues regarding running RAM at higher speeds than it should.