The problem is that for the new cards R9 280x you need the latest Catalyst Drivers. 
.
.Uninstall those drivers, then run amdclean.bat which is:
cd C:\Windows\System32
del SlotMaximizerAg.dll
del SlotMaximizerBe.dll
del amdocl64.dll
del OpenCL.dll
del OpenVideo64.dll
del OVDecode64.dll
cd C:\Windows\SysWOW64
del SlotMaximizerAg.dll
del SlotMaximizerBe.dll
del amdocl.dll
del OpenCL.dll
del OpenVideo.dll
del OVDecode.dll
rmdir C:\ProgramData\AMD\KDB /S /Q
Then install http://developer.amd.com/amd-license-agreement/?f=AMD-APP-SDK-v2.7-Windows-641.exe
Then do a selective install of the latest Catalyst drivers, making sure you don't install any more AMD APP SDK drivers.