Having the model of your graphics card, and the rest of your spec would likely be helpful. Your problem *sounds* like a memory, heat, or power issue.
Heat is the easiest to check. Download CPUID Hardware Monitor and watch your cpu and gpu temps while running stuff. Download Furmark and run a torture test on your gpu. Can it run for 15 minutes? What temp does your gpu reach? does it stop climbing
at some point (eg 75C or 85C)? or does it just keep climbing?
If you can run Furmark for 15 minutes, you've likely eliminated gpu heat and gpu power supply as problems.
You can then quickly run Windows Memory Diagnostic. Simply type those words into the search block on the Win7 Start Menu, and select the program when it pops up. Follow the instructions, and watch the screen during the entire test. If there's a problem detected, it will first be noted on the blue screen, and (maybe) later if a summary appears in a buble near your system tray on the next boot.
There are other, better mem tests - but the Windows one is easy and surprisingly effective.
Let us know what happens.