Download antimalware and see if your anti virus let escape something.
Download pograms to test your ram, cpu, gpu, drive...
If everything its ok... just reinstal windows
-high end pc
downlaod speedfan and see if cpu or gpu overheating and see if something is writing on your harddrive, that can fuck fps
make sure you have multi-core rendering enabled and have -threads 4 in launch options (-threads 8 give less FPS for some reason.)