Got a GTX 670 when i bought it it was working like a charm
Turn on cpu one day my monitor is filled with rainbows and distorted crap
Take out the 670 ...put in my old 560ti
Same thing rainbows ...
take out the 560ti put back in the 670./.....more rainbows
I put the 560ti .,.....Success my persistants pays off and the 560ti works ......now for the past several days here is what i'm not understanding ,,.,, I play wow on the 560ti all is good i turn off the cpu for the night
when i go to play the next day ....RAINBOW HELL
Take out 560ti put in 670 ---Rainbow hell again put the 560 ti in .....all is well and i can play again
Now 2 questions .....why is the 670 giving me rainbow hell ....and why do i have to put in the 670 then take it out and put in the 560ti to make the 560ti to work?
Tried updating drivers of course I have no idea what else to do ....ultimatly i want the 670 working again but meh whatever i can live with the 560ti if i dont have to keep swapping them in and out to make them work
Perhaps the PCIe lane is dying. If you have another lane, or barring that, another computer, you could try putting the video cards in one of those and see if the artifacts persist. If the artifacts are gone, then it may be an issue with your motherboard. Another possibility is that your PSU isn't supplying enough power to the video cards, although I would think this would make it blue screen and not artifact.
A third thought is that the video cable you are using may be damaged. Perhaps a pin broke off inside the 560 ti, and the cable is only having full contact when you are using the 560 ti, and only some of the time.
Id recommend completely cleaning your drivers and installing the newest ones. If the problem happens again, completely remove drivers again and use and older revision of the drivers. Drivers could and would cause issues on both if the revision is not that great. Alternately, we have a few possibilities including above mentioned theories and some others id like to research more before saying anything.