Do you mean it flickers, or shudders? That could just be a problem with your refresh rate. If that's the case, check your monitor's documentation to see what refresh rate it supports; in most cases, it'll be either 60 Hz or 75 Hz. Try pulling up the settings menu on your monitor to see if it has a "sync" or "refresh" function.
Otherwise, you may just want to go through the standard troubleshooting steps. Uninstall the ATI/NVidia drivers through control panel, boot in Safe Mode, clean with Driver Sweeper (http://phyxion.net/) and CCleaner (http://www.piriform.com/ccleaner), then visit the card's official page and re-install the drivers.
It might also help to re-seat the video card (if you aren't using an integrated graphics chip) and your RAM. Making sure you have all of the proper prerequisite software (such as .NET frameworks for the ATI suites) is always helpful as well.