Hrm, never used OCCT before (had to google it to figure out what it was). Does it have an actual 3D rendering it displays at it benches, or does it just do the test and display results without a live 3D scene? I can't find any screenshots that aren't just interface, so figured I ask.
I had some similar tools back in the day that would do artifact testing, but weren't always that great for detecting problems by themselves. But, that was something akin to 10 years ago and I haven't used anything since. And since I haven't used OCCT (and am at work), I'm just kinda tossing out ideas.
Hrm, another random thought. Maybe the VRAM is overheating a bit in games, so you only really notice artifacting during high GPU use? *shrug*
Mabye fire up one or two other games just to be sure, but I'm leaning to a hardware issue. You can always clean the drivers and OC apps (Afterburner, et al), install fresh, and try it at stock to rule out something with the drivers or a 3rd party tweaking tool.