OK just a little info for future reference: The R9 280X is actually the same Tahiti chip as the HD 7970 just binned a bit higher for extra performance. These chips run HOT! Because the toxic coolers pump heat into the case you need to have a way to force cool air in and suck the hot air out.
Looking at BitFenix's page on your case it only lists 1 fan included and that is on the rear. If you add a 120mm fan to the front this will help tremendously with pushing cool air in and help force out the hot air.
But first, only because this can be done now, Run your game that cause's the issue. Also run a monitoring program in the background to log temps and usage of the components. Once you notice the performance drop, in windowed mode, look to see if the CPU frequency is dropping or if the GPU frequency is dropping. Doing this will help point to the problem component.
Sapphire has an OCing program that will help with monitoring the GPU. it is called TriXX. This is the same program as MSI afterburner, EVGA precision X, HIS iTurbo ect.. They are all Riva Tuner programs with a different skin so any one you prefer will work just as well.