Have you tried running just 3 280x?  Are you undervolting? Are they stock volts 1.2ish?   Which CPU are you using?  I know intel, but which chip?  Which OS?

A stock volted / stock clocked 280x under load will pull 275-300w minimum.

If you are not undervolting i can virtually assure you 4 280xs + system are a bit much for any 1200/1250w unit, platinum or not.

The odd power cycling issues you describe would appear to indicate your PSU is balking at the load.

Generally, latest beta drivers / 13.12s + 2.9 SDK work well with the 280xs.