Question: Is it hot to the touch when it's acting up. If so, two possibilities. First is the onboard memory is bad or going bad, it can happen due to over heating. Second is overheating caused by air flow blockage of the heatsink inside the GPU and dirty fan.
If you not too apprehensive, take out the GPU and take it apart and check out the inside of the unit. Then with it completely broke down, give a thorough cleaning and reapplying thermal-paste to the chipset. It's a great learning experience.