It's difficult to say amonra666.. You can look into this thread,
Gaming issues with 1.5.13 driver, for more information about gaming issues on these new 5.13 drivers..
As for your stability / overclocking issues, I'm not sure, overclocking can go smooth or rocky (as we all know)!! Your glitches may have nothing to do with your overclock, or it could be that your overclock damaged your card (especially with almost 200mhz on your core, geez).. It does not really sound like artifacts, so I would lean towards the first suggestion..
Honestly, it almost sounds like a VSync issue. Though, with VSync turned ON it may correct the problem, you cannot overclock your card (as it can only render 3 frames ahead anyways). It may be that you are overclocking, and VSync was turned on by default with the install of these new drivers.