If your refering to the core clock on the video card then it sounds just like it was being downclocked because the core was idle and if you were running ooct at the time then something isn't ight and it may be a driver problem or defective hardware.
You can start by removing the current drivers and reloading them and also making sure that you have the current drivers. It would also be best to not have beta drivers and to have WHQL drivers loaded untill you discover the issue.
Your power supply if working correctly is enough for the components that you have but if there is something defective with the psu and it's not supplying the correct power in the pci-e cables then that could be an issue. You don't seem to have an issue with your cpu and if the psu was overall defective it would affect the other components and not just the video card. So if the psu is supplying the wrong current to the pci-e cable that could cause your problem but something like that is hard to verify and is quite unusual. Most likely it's a driver issue.
Ok so im now overclocking the memory clock and i noticed something werid when i was running ugine. I was seing minor drops in the gpu usage which whould usually corispond with an artifact, but when i put the memory clock back to normal i was still seing these usage drops. In fact when it was at normal i saw the biggest usage drop yet when it droped from 99% to 40%.
Keep in mind that i had no overclock on the core clock, i was only overclocking the memory.