Could be a driver problem - I'd recommend running a driver sweeper and then reinstalling your graphics drivers.
Could also be bad hardware, or some other sort of software glitch. I'd recommend checking your Windows error logs (right click My Computer --> Manage --> Event Viewer --> Windows Logs --> Application/System) and see if you have any warning or errors in there that might explain what is going on.
Worst case scenario, you could just disable sleep in your computer. An old rig of mine was always unstable when Windows 7 would come out of hibernation, so I just disabled it and problem solved.