I have two sets of DDR3 2000 2GBx3 Kits. Mushkin redline and GSkill Trident.

When they are used together, they all are picked up by the board up till 1866mhz. anything above 1333mhz requires vtt/Qpi/Dimm to be set to 1.55v and vdimm to 1.65/1.66v. Past that, board shows anywhere between 4 and 10 GB depending on its mood/setting. I was able to get them to play nice first try for a while at ddr3 2000 but now, no matter what setting/voltage, I can't. Might it be a cpu, mem, psu, or board issue?

Board is a p6t7 Asus with an i7 920. PSU is an HX1000 and i have 3 GTX280s in the rig.