this is an issue that does require some crafty repair. what you need to do, it on the logic board, remove the heatsink for the video card, and re-apply thermal paste, then, when you re-attatch the heatsink, put it on extremely(But Evenly) tight, this will resolve any joint errors, once that is done, that may fix the problem. if not, if the video ram is intregated in to the logic board, make sure that all pins are making contact, because this can be a video ram related issue as well. if the video ram is shared, then the ram chip may be bad, hope this helps,
Chris