Dr. Eatsome

I am currently using this card and well, it is good, but the problem is when i play games such as call of duty 4, and i use call of duty 4 as an example because that is the game i've been having this problem on the most. Basically i'll get on, join a server then alt+tab out of there to use the ati tool to speed up my fan to keep the temps low, then i go back and start to play, and everything runs smoothly, no lag, textures are good, etc etc, but eventually the screen will lag then cut out, when it cuts out it will usually say no signal and input d sub and usually my screen recovers from this and i go back to the game, and other times it just freezes and i have to restart my computer manually. At one point today i had the fan on full blast to see if it was the heat and i would check every few minutes to see the temperature, then go back to the game, and it was hovering around 62-65 C, so i went back in, sorta threw a gernade and the screen cut out and froze, so i had to restart manually. So what could the problem be?