hmm your GPUs might be throttling. or it might be a stability issue for the same reason that putting a lot of memory stick on a motherboard can also lower stability of the whole rig.
If the GPUs are throttling it's not due to temps -- they are all <70C.
EDIT: I'm no electrician, but I thought I'd use my handy Radio Shack voltmeter to check my household current. Seems like most rooms have about 117 volts available, but the room where the miner is only has 108 volts. Is that voltage drop significant enough to cause this problem?