garbled display

At times I have a red ‘grid-like pattern’ on the monitor. It happens when they play a game. I can get it also when I run a full test at PcPitStop.com (everytime they get to the video test). But they do not pick up on a problem with the function of the video. I have changed drivers for the card several times including downloads of new drivers. The card is ATI Radeon 7000 64mb. It did not come with the computer.