Thanks Alan,
Cecking the “Don’t use texture RAM for BG Images” under -Preferences/Open GL Tab- fixed the problem.
Just to confirm: In my case the lines would only show up with a BG AVI loaded.
It did not seem related to the number of emitters added, and if I removed the BG image the lines would not be present in a final render featureing just the emitters.
Any technical insight into this?
Ray