According to that screenshot, your video card is quite hot, while your CPU and MB temps seem pretty mild. Case fan airflow might help you, but the rest of those temps make me think it's actually fairly cool in there.
I'd focus on the video card itself. Are you overclocking it? Does the videocard fan still work? Does it work but is extremely dusty? Does it still have good contact with the core?
After inspecting things, and using an air can to clean up, the next thing I'd try is taking the video card out, removing the fan and heatsink, cleaning that all up, putting some good thermal grease on, and re-seating the video card's fan and heatsink.