I pulled a card out and it's working fine. Didn't bother testing with both cards and no crossfire.
Crossfire was working fine on Windows 8.1.
One other useful bit of info: I have a rev1 and a rev2 r280x. Hasn't been an issue to date, but maybe is part of the issue?
I'm having the same issue with a similar build.
240GB v300 kingston ssd
2 hitachi 7200 rpm 500GB Raid 0
280 x2 (one disabled)
evga bronze 750w psu
msi z77a-g43 motherboard
crucial ballistix 8GB 1600mhz (x4 = 32GB)
When I enable my second R9 280, fullscreen games either stutter horribly or crash all together. I have tried clean installs numerous times in different orders, I've done it with and without security enabled, I've tested both pcie ports as well as both cards, etc. Both give a signal that they are working. The only reason I can come up with for crossfire not working is that either a) drivers or software are/is buggy on AMD's side or b) the difference in BIOS Part Number. I really hope it's not b since that is really stupid. If that is the problem, it would be nice to know how to flash the correct version onto it. That is literally the only difference between my two cards.
Just tried a flash to the correct bios (seems that whoever owned it previously flashed it) and while it didn't mess anything up, it didn't fix anything either. The only other difference I see is in Crimson System Info tab, it says one is AMD Radeon R9 200 Series #1 and the other is Series #6 (said this pre bios flash as well). Everything else on that menu is the same.