Thank you so much i think im on the right way now to solve my problem. I do experience alot of audio dropouts especially when opening Frequency Plugin or anything that shows Spectrum curves. Audio dropouts stop once i tab out of cubase. Using windows graphics driver seems to be the only working way so far. I installed radeon driver (without software) and changed the reg key. but still i have dropouts 
Do you by now have any further information on that topic?