So, your gpu is fine and you should keep it.
Your cpu is fine but for some reason in underperforms(can't talk about ACO, but gta and forza shouldn't stutter),
Possible fixes:
Task manager to see what else uses cpu while you are gaming and closing those,
Checking the temperature and clock speed while you are gaming, if the clock speed is under 3,8 or 3,9ghz then the problem is most likely the cooling,
If nothing is running in the background, the temps are fine, clock speed is fine, then you should try reinstalling windows.