It's the first time I got this problem. When I render to .avi file, big "electronic squares" appear in my rendered file. I tried three times, defragmenting, shutting all apps around (including Norton Anti-Virus) but these squares still continue to appear, evertime almost on the same spot... Why? This is time consuming since it's one hour and a half of rendering every time I try, just for a 5m.30s. video-clip... Now I'll try this: resample (check box) each and every clip into the problematic region and render... if this doesn't work, I really don't know what to try next...