Cause of the distortion in the XT1900
I freaked out when I first saw the distortion; it looked like someone had put a screen over the monitor. It went away overnight but came back the next afternoon. Time passes, no problem, then it starts again much later.
Difference, room temperature. If the inside of your Mac pro gets above about 27C, you will start seeing the distortion because the on-board cooling is inadequate. My solution has been put point a small desk fan across the exhaust area, but that's obviously noisy. I was researching a replacement when I ran across this post.
For you conspiracy buffs, I actually had put in an order in for the ATI RADEON HD 4870 512MB-ZML part no. in early September, which was expected to ship at the end of September. When I checked, the order had been cancelled on September 22, about the time the advisory came out. When I called about the order, I was told that the order was cancelled because the part had been on back order too long.