I am overlaying video on top of video in Premiere 6 via transparency. When I do this, render, and play back, a very slight grid effect appears on the picture. This problem is only slight, but is noticable. What I mean by a grid effect is small cubes covering the entire picture. The picture itself is not pixelated. Hard to explain, but I am hoping someone knows what the cause of this problem is and how to solve it!

Many thanks in advance.
mrobbo