Hi, just see this topic. I've installed the same video mod and I have the same issue:
http://atariage.com/forums/topic/280489-betagamma-computing-purevideo-2600vec-video-quality/
Did you solved this problem?
This is a Sunnyvale Light Sixer PAL, from UK, used in Italy on a Panasonic Plasma screen. The tv supports both pal and ntsc so this is not the problem.