The first thing that comes to mind is a faulty video card. But you should first confirm: Your power supply, is it a good PSU providing enough power?
Your drivers, have you done a clean install of the latest drivers from the Nvidia website?
Cables, is everything connected and is the video card seated properly?
Since it seems like an issue when the card goes into 3D clock mode, it could be power or a defective card. Does it only happen when starting a game?
One thing to try is to go into the Nvidia Control Panel and set 'Power management mode' to 'Prefer maximum performance'.