I recently bought myself a Red Devil 6900XT and was puzzled by the same problem. After numerous attempts to find the answer, a concluded that the problem is massive, related to the driver. I have set up manual configuration management in the Performance tab. And I set the maximum GPU frequency as the one specified in the characteristics of my card (2340MHz instead of 2504MHz by default). After that, the video card was stabilized. I can only advise you to do the same thing, only with your maximum frequency.
