I've got four running with 7500:880, getting ~440GH/s on btcguild, power draw of three of them is 410W at the wall according to the watt meter I have. Though I don't trust the watt meter much...
Oh, and they have dropped off once in the last 4 days. But that may have been when I added the fourth miner.
Happened a lot to me lately with different miners. Measure your 12V line - it should be 12V (not 11.9) under load. Consider replacing power supply! I use IBM server 825W PSUs and absolutely happy with them - 12.1V under 100% load!