can you list what GPU you have the 3 things I'm aware that can cause the issues you are having right now.
One is a a bad driver install in which you need to do a clean install meaning you delete all traces of the old driver.
Second is one or more of toy components are over heating in which case you can use your favorite monitoring software to check the temps. The temps can be from dust build up thermal pads/ paste drying out, bad airflow.
The third is if you tried to use certain shadders and or AA setting from sweetFX.