Iirc isn't it first year OcUK then his after that his? Does it happen all the time or after a long gaming session? My 7970 would do similar when I had it overclocked - someone said it could be VRM temps creeping up. Try cleaning out the cooler with compressed air and making sure you have enough air getting into your case...
Started happening in GTA after only 10 minutes or so, case has pretty decent airflow with two 140mm intake fans. Even had the side of the case off the last couple of days and the card was cleaned recently.