jagabo really helped with the mpeg2modifier recommendation.
Used mpeg4modifier to make the AVIs packed bitstream and all problems went away opening the avis with avisynth with avisource. Even an avi I had problems with which was supposedly packed bitstream I tried to unpack then pack again and hey presto it worked just fine - go figure.
Is there any other ways to open AVIs the "non-VFW" way ? I'd have thought problems with xvid AVIs and avisynth would have been more common. Seems like a bit of a "hole" in avisynth in the modern environment and I'd strongly prefer avisynth remain relevant
