Hi, I happened to have the same kind of problem, but I managed to fix it.
My GPU usage started jumping from 100 to 0 and back ridiculously fast after a few minutes of gameplay. The problem is that CPU is throttling down. I didn't see a reason for this because I have my fx-6100 OC'd at 4k, and my cooler is beefy enough (antec kühler 620). I found a BIOS setting called "AMD Cool n' Quiet", I disabled it and now my PC runs like a dream.
So, I would suggest searching BIOS for "CPU Features" or something, and checking what options do you have. There should be at least something that limits your CPU/CPU Cooling.
Let me know if you find success