As Carazon pointed out, it could be your PSU. What is the recommend minimum PSU wattage from vendor. If not enough power from PSU, this will cause problems. Have you downloaded the latest drivers from AMD? Check these out and then will go from there.
It could be an overheating issue too, check the fan area and card in general to make sure it's clean from lent and dust and any debris that might be stuck some where inside. Check with speed-fan or SIW, both are free, to see what temps your card is running at. You can also check with in CCC panel.