Did you inspect all boards for BAD CAPS as well as clean all heatsinks an replace all old heat sink compounds?
I have a zotac gefore gtx 560 that has always given me some trouble with visual artifacts. At first I assumed it was my old power supply so I upgraded to a new one and it fixed the problem 90% of the time but it still happened occasionally. Now almost a year later the problem has returned and I see them all the time. I ran a
GPU stress test and I got several errors.
The fact that the exact same thing has happened with two PSUs make me feel like it is the card. Is that a good assumption?
GPU temps are not out the ordinary and it is running at stock clocks
this is my psu: http://www.newegg.ca/Product/Product.aspx?Item=N82E16817152028