first try vdimm of 2.2 to 2.3v to see if that helps with all four sticks...think about air cooling the memory sticks.

what is your "MCH" (memory controller hub, AKA, Northbridge) set to?...you might need 1.52, 1.56 or 1.64...don't forget the Southbridge, AKA, "ICHIO1.5" will need extra help to communicate to all four of those memory sticks...i'd recommend 1.65 to 1.7v, at least.