avi doesn't tell us much of anything...since it is a container (not a format) there can be many codecs that can produce avi files.
get gspot from the pic in my sig, drop your video files into it and post what codec it says (codec and name...or just the name)
if it's divx or xvid...that's your problem. You should recompress your footage using a lossless codec such as lagarith or huffyuv
in that case, read this
followed by this