The post above explains the initial issues. After downloading driver sweeper and installing new drivers, I found that the issue did not go away. As soon as I enter a game, the screen will go blank. I decided to reinstall ati tray tools and found that the problem was immediately solved. (The screen blanking). However, the issue with the GPU running slow was still persistent. That is when I realized that the Ati tray tools "under clocks" my GPU (Radeon 6850) to 300Mhz Clock speed and memory speed. Thinking that I've resolved my issues, I immediately turned the clock speed and memory speed back to 775Mhz clock speed and 1000Mhz memory speed. However, as soon as I enter a game, the same issue occurs from before installing ATT. (Screen blanking).
I experimented with different clock speeds and found that:
@500Mhz Clock Speed, 600Mhz Memory speed, I was not able to recreate problem.
@650Mhz Clock Speed, 850Mhz Memory speed, the problem occurs after ~5 minutes of gaming.
@776Mhz Clock Speed, 1000Mhz Memory speed, problem immediately occurs
This has led me to think that perhaps this is a problem with the PSU supplying power to the GPU?
Are there any more tests I should try before looking into replacing parts?