Normal FPS, stutter and unresponsiveness in games that rely on CPU, but low CPU usage (10-20%), normal temperatures.

I'm running windows 7 using GTX 750ti and AMD fx6300

On games that rely mostly on GPU I get good performance but on games like CS:GO my mouse feels unresponsive, the screen freezes for a couple of miliseconds every row and then even when running the lowest possible settings and a low 4:3 resolution. The computer isn't dusty. I'm on a stock cooler, but the temperatures seem normal. Thermal paste hasn't been changed in 2 years, could that be it?

I was very used to these problems and blamed my low-end CPU that isn't good for CS:GO, but what suprised me the most is how a couple days ago I switched off AMD Turbo mode and 'Cool 'n' Quiet', cleaned the computer from dust and suddenly CS:GO ran like never before, smooth and clean with no problems. The next day it was back to how it was before. Since then I've been trying to investigate the causes of my problems. My motherboard is too bad to OC, my bios is out of date, my nvidia drivers have been cleanely reinstalled today, my nvidia control panel settings are for high performance, power saving isn't turned on, my GPU is also not on heavy load (~30%). I've looked for answers but most of them have no solution even after being asked a couple of years ago. Can anyone give me suggestions of how I could regain that smoothness I got to experience for a single evening? I would be very grateful!