Ok, I found that by using a sequence of png images and rendering in mental ray, the weird lines are gone. Thing is, I can't get the sequence to play. I checked the "use image sequence" box, and tried setting keyframes to get the sequence to play. I want frame 1 of the sequence to play at frame 101 of the scene and frame 115 at frame at frame 215. It just doesn't work. It worked when I tried a bitmap sequence, but that had the same lines as the avi file. This is kinda weird. I never had this problem in earlier versions of maya.