My post probably comes to late, i just wanted to share my success story on repairing a broken, artefacting  EVGA GTX780/6GB using a simple, small TRAVELLING hair dryer ( NO oven baking involved ). Originally i had put my comments under this youtube video ( w w w.youtube.com/watch?v=30nXhkvY9E4 ), because it shows the same initial problem i had ( but which went worse to totally nonfunctioning soon after )
 
So in short: out of the blue my expensive, but great EVGA GTX780-6GB showed those artefacts after 3,5 yrs in service ( so no more warranty ). My card never had much stress, as i mainly do 3d work on it ( 3ds Max , unity ) instead of  gaming. So maybe the lack of regular "heat" caused the lead of the GPU's soldered contact grid array to become bad ... who knows .
Anyways, it started artefacting out of the blue and after a while, the card totally broke, showing artefacts even at the BIOS screen. and windows did'nt even boot ( deadlocked when it initialized the graphics driver ) Tested in different computer, same problem.
But now comes the fun part: I read up about people putting the card into the oven and "bake" it at 100-200 degrees celsius for 10 minutes to repair the problem. did'nt want to do this, but nevertheless i stripped away all fans, cooling plates and other cooling material, so all that was left was the pure naked electronics board with the GPU on it. To be sure i even tested the card in this state in the computer, still same problem at BIOS screen Now i used my small traveling hair dryer and let it blow over the GPU really close for 5-10 minutes, slowly swiping across the GPU chip, back and forth. This way the GPU really heated up quite well ( i tested with my fingers and it WAS burning hot, but probably not as hot as when i would have put it into the oven ) After that i plugged the naked card into the computer again, and guess what ? The artefacts were gone at BIOS level. Confidently i reapplied all the cooling equipment ( had some thermal paste at hand, thats of course essential ), put the reassembled card into the computer and switched on: Card works perfectly fine now, no artefacts, not on desktop nor in high demanding games This was a great DIY experience, as i already was prepared to shell out about €550 for a new shiny GTX1070TI Now the trusty GTX780 still will suffice for some time, but as i read, sooner or later ( few months ) the problem will reappear, as its more a temporal than an ultimate fix