resurrect: Ok here's what I can see could be an issue, your PSU isn't powerful enough to handle the 2 R9 280x cards, you should have at least a 800w in there, (I know for 1 R9 290 they say the minimum PSU you should have is 700w). just hit up pricespy I know there are 1kw PSUs for under $200.
Update: PSU tested, ram, all tested ok,Unable to fault machine (They don't seem to understand the term intermittent) Replaced Motherboard (Possible Incompatibility issue)
btw, I thought that to BUT, they only say that to cover themselves from customers using cheap PSUs that arn't delivering good power,
Plus I don't have heaps of hdds in my computer my computer could actually run on a good quality 600watt PSU..