|He has 18 GB's of RAM. Not space. So, ignore that guy ^.|
Windows 7 is probably not the answer.
When you are gaming, is it a particular game or any games you play when it does this?
Is your computer getting noisy when this happens?
Have you gone to ATI's website to make sure you have the up to date drivers for your video card? Windows Updater won't find those for you, you have to go look yourself. Go here and download the up to date drivers and CCC software http://support.amd.com/us/gpudownlo...
To me it sounds like either your hardware is overheating or you are having a software issue. However, the software issue seems more unlikely since this is happening after hours of gaming not right away. That lends itself to the idea that this is a hardware overheating/stress issue. One easy way is to check your GPU temperatures using a software like HWMonitor. Post your temperatures when you turn on your computer, and after 1-2 hours of gaming, and each hour after that. Make note of your systems noise.
Also list your operating system and computer specs (CPU, etc.). Download CPU-Z and run it to get all the info.