Do you get a kernel panic? If so please post the results here....
What did you test the ram with? If the apple supplied test, that isn't always 100% so I would pull as much as you can and try to crash it, then replace what you had with what's left over and try again.
If it still crashes, take it to apple and have them verify, with their own eyes, it crashes when you encode an MPEG@, don't accept a replacement unit until it STOPS crashing.
I had a similar problem with a DP 2.0, had to make them see the problem, and when they called me to pick it up, they plugged it in and verified the problem was gone (after 3 mainboards) sucked but now it works 100%
