Just off the top of my head, it sounds like the GPU is running too warm. I don't know how comfortable you'd feel with disassembling the machine, but if it were mine, the first thing I'd do is remove the palm rest. Once it's off and the system board is exposed, I'd use some compressed air to clean the exhaust ducts, hold the fans still and clean them thoroughly.
Once it's all cleaned up and dust free, if the issue persists, I'd run an Apple Hardware Test (AHT):
Intel-based Macs: Using Apple Hardware Test
If that comes out clean, my next step would be to reseat the video cable that runs from the system board to the screen - and replace it if it shows signs of wear or damage (it will flex with the screen each time you open/close the lid).
Hope that helps. If you're not comfortable with working on the machine, you may need to seek professional assistance.
