Good man, that was going to be my next piece of advice. The clock fluctuation issue is not present in 15.11.1 Beta or earlier drivers. We hope to have a fix for this one soon, so stay tuned!
Thanks for the info Matt. At least I now know it's not a fault with the hardware. In the meantime I am happy to keep the 15.11.1b drivers installed as Dying Light The Following is the game I am currently playing through.