Sounds to me like it is simple, that silent pipe does not perform AT ALL
RMA it and get a card with a FAN on it. PCI-E 2.0 IS backwards compatible FULLY and this is not the issue and has nothing to do with it at all.
Some not all silent pipe cards have similar issues, sounds as if you found one of the not so good choice ones. Artifacts and stripping is memory and or core overheating, and for you to see it already at windows loading screen is for sure not a good sign for that specific card model
At the very least if you cannot get a different model, put a FAN on the heatsinks
Good luck, and Please dont take what I say the wrong way. it is just how I see things, and it is based on your comment and my previous knowledge of GFX cards and silent pipe performance, and artifacting due to heat issues