I finally got around to try it out in a different computer - better late than never
and it yielded som interesting results.
Firstly, I upgraded the driver to 175.19 which did not solve the issue. If anything it seems to occur earlier than before but I have not been able to verify this. It still looks something like this (props to USFORCES):
I tried inserting the card in a Dell precision 390 workstation with the same result. I noted the temps with SpeedFan 4.35 when the flicker started to show:
Running 3DMark gave a score of approx. 4600, but with no errors or flicker. Running Memtest did not yield any errors either. Very strange...