Household Voltage Sag -- A Problem?

[mikeo]

I bought this motherboard because it had 2 x16 and 4 x1 PCIe slots with visions of a 6 GPU rig. However, I'm having trouble maintaining stable hashrates with only four GPUs. Here's the setup:

New Antec HCP 1200W PSU
MSI 890FXA-G65 motherboard
AMD Sempron 140 CPU
Kingston 1GB DDR3 RAM
2 x 5830, 2 x 5850 GPUs with PCIe extender cables
LinuxCoin 0.2.1b persistent from a 4GB flash drive

I'll startup the GPUs one at a time (they are OC'd at 975,300 for the 5830s and 875,300 for the 5850s) and the first three hit expected hashrates of 308 MH/s for the 5830 and 355 MH/s for the 5850s. As soon as I fire up the 4th GPU hashrates fall off and range from 225 - 308 for all four cards and fluctuate dramatically.

While this seems like a voltage issue to me the rig draws only 620W max, but it does fluctuate over 100W. The GPUs are plugged into the 8 x 6pin PCIe PSU cables, the 8pin CPU power cabe is plugged into the M/B, and I'm using the MSI supplied 6pin-to-2xMolex adapter (plugged into 2 separate PSU molex lines) for the M/B PCIe power.

Any ideas?

[haploid23]

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.

[mikeo]

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?