Stuttering for no reason

so yesterday it was fine i can play grimdawn and get around 40 ~ 51 fps

but today when i boot up my PC i get stutter so bad its become arround 17 ~ 21

i allready try to defrag turn off antiviru etc the only way to make it doesn’t stutter is going ALT + TAB to window and back to game but it doesn’t even take a long to start stuttering again

any1 got a fix for this?

Intel i5 3.0 GHz
NVIDIA GTX 960 2GB VRAM
8 GB RAM
Windows 8.1 64 bit