Is my graphic card the culprit?

My computer has a peculiar problem that I have not found a solution for in a year's time. Before my new PSU that I have installed, anytime I would use Photoshop CS2 I would work fine on it for about 15 minutes or less and then my screen would freeze and it would then shortly go black. My monitor would say to check cables as if it was not connected to the graphic card. 2 seperate monitors later I still had the problem. I figured the power supply didn't have enough amps on the 12V rail so I bought a new power supply with plenty enough wattage and a total of 35 amps on both 12V rails. Alright where here's the hum dinger... now it dosen't give anymore problems with photoshop but whenever I scroll too quickly or too much while surfing JUST the internet, it will do the same thing all over again. I know the computer is still running because my music will still be playing without any skipping. My graphic card is a Sapphire ATI Radeon HD 2600 XT 512MB DDR. If there are any solutions or answers to this problem PLEASE PLEASE help. I am at my witts end and I am very appreciative of your time.