The 320.xx are known for having tonnes of issues. Even reports of frying cards.
Monitor the GPU frequency + temperature + fan speed (I found 320.11 was OCing my card without increasing fan speed)
I think 314.22 is the last decent driver for most people. (Will need a clean install, the 320.xx seem to be a bitch to uninstall)
If the artifacts continue, then I would RMA your card (it's probably been damaged).