If it is fixed by a restart, then I am almost positive the problem is down to drivers. Have you tried a slightly older version to see if the card still clocks down?
Maybe even try reinstalling the drivers. Uninstall the currently installed driver, ensuring the folder is also removed (I find driversweeper works well, and I bet there are a few other programs that do it too) and reinstall the latest version.
Also, does it downclock whilst an overclock is applied? If not then you can just overclock it by a small amount, or even more if you want to see a performance increase - MSI Afterburner is great for this.