I get similar issues when I push a GPU overclock too far.
Are you overclocked?
They are stock overclocked. As I have in the past, they are not currently overclocked.
The other thing I would check is heat, but that usually just causes the GPU to throttle back, rather than causing artifacts/scree tearing.
You could try manually "underclocking", but if that does fix the problem, it will just confirm that one of the cards has an issue.
Then it will be a matter of using one card at a time until you figure out which one is bad. (Going bad)