Crossfire 290x low fps Battlefield 4(after a bit)

A CPU Bottleneck wouldn't cause you get get worse framerates.

I think it has to strictly do with thermal issues. Try changing your airflow to get more air out of the case, and find some way to strap a fan onto your VRMs.

I don't have that build anymore. I did fix it with a lot more fans though lol

it was a cpu botleneck like i allready said,
and yeah throttlei i also talked about that, its kinda an issue with that mobo you had.