You may need to lower your overclock on that particular GPU and/or raise the voltage (if you undervolted it) to fix the invalid shares issue. Try reverting it to stock settings for testing first of all.
All gpu´s are in stock clocks and not undervolted.
With win7 everything was working 100%
Hmm, if you're at all stock settings then you're using ~250 watt per GPU? You might not have sufficient power if that's the case since 7 x 250 is 1750 watts just for GPUs...
Hey, good point, havent even think about that, have to learn how to undervolt those