Each 280X requires around 250-300 watts of power. At 12V, this is 20-25A per card. Based on your power supply specs, none of the 12V rails can supply that much power. You haven't indicated how you have the power connectors connected, but to ensure that the maximum amount of power is available from each 12V rail, you could try using molex-to-pcie or sata-to-pcie connectors on as many different separate cables as there are coming out of the power supply. Unfortunately, it appears this power supply was designed to provide power to power-hungry CPU's as opposed to GPU's, so this still may not work.

The ideal solution would be to get a different power supply that has 12V rails capable of providing 25-30A each, or has a single 12V rail that can provide at least 60A. You could also try undervolting the cards to use less power.