Graphics performance issue

Ever since I installed Windows 7 64-bit on PC which used to be Vista I've had performance issues with my graphics card after I wake my PC up from sleep mode. I have an ATI Radeon HD 4850 card and I have all the latest drivers so it's not that. I'm not sure what it is wondering if the people on this forum can help me out here.

What happens:
After my PC gets waken up from the sleep state it's like the graphics card lags. The Frame rate on everything from videos to 3D games drops to around 1 or 2 and sometimes videos just freeze and don't play (Even though sound keeps going). I've never had this issue with Vista it only happened after I installed Win 7.

Hi Thewon,
did you upgrade when you swopped over to 7? If so, try a clean install..

At first I did do an upgrade but afterward I did a clean install to see if it would get rid of the problem but it didn't.