If it seemed to fix the problem by lowering the GPU clock, that points to it being a thermal problem. Find guides on youtube or the internet on how to clean your hardware safely. Clean the card of dust very carefully. Make sure the fan spins freely and is working well by using a utility like Hardware Info 64. You can see all types of info on Hardware Info 64, make sure it is within range for your card. You could re-paste the GPU and checkout the thermal pads on the memory if it has them. Again check online videos or guides. Always be very careful when doing these, take your time and follow the guides well. I hope you find a solution.