grainy/unreadable LCD that works when connected to another computer

I am currently working on a Power Mac G5 1.6GHz with a Radeon 9600 video card (64MB) and ViewSonic VA520 LCD. I've had my current computer/display setup for almost four years now without problems. Lately, the display intermittently becomes grainy and unreadable and then will suddenly clear itself only to repeat the cycle. I have hooked up the display to another computer and discovered that it works properly on the other computer. It would seem that the problem lies in my graphics card or software. I have reconnected all cables and tried to remove any device that would cause interference with the monitor only to get the same results. I have found that upon restarting the computer, the screen will be clear at startup only to become grainy and distorted a few minutes after. Can anyone shed any light on this?