Low FPS and fluctuating GPU usage in CS:GO?

Hi

I recently sold my brother a powercolor 390, but for whatever reason he is getting lower FPS in cs go with it than he did with his 750ti.

We opened MSI afterburner and the core clock wont go bast 400-500MHz and the GPU usage goes from about 5% then spikes to 100% repeatedly. I don't know what the issue is. He has a 4690k and none of the cores are going above 70% usage, and other games (we tested hitman and 3dmark) and they both got up to 1010MHz, the stock clocks. I have no idea why this would happen. I tested it in my rig on my temporary card (780Ti) and it's working fine, getting up to the stock 960MHz.

Edit: he has the newest Crimson drivers