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.
ive got 1 rig running downstairs on 4 cards atm,
so it should be working
right now on this rig im having trouble with the graphic card drivers
everytime im in computer management it would Crash my card and say "stopped responding but successfully booted back up etc"
im not sure which drivers to install
this is all on the windows 7, i guess i followed a different install on my other rig. im using a undervolted cgiminer.