The reason I ask is that they sent it back to you, but they could not replicate your RAM test failure - correct?
Therefore, I conclude there is something about your system that isn't interacting with your RAM properly (possibly a slot on your motherboard).....whether thats the cause of the graphics glitch, I'm not entirely sure yet.
However, lets get the RAM issue out the way since you did have errors with that. Run MemTest86+ : we recommend a minimum of 7 passes (typicaly an overnight job), or until you get errors.
Post back here when its done and we can take it from there.