Yes - it seems like morgan has faulty YV12 output. It seems like they fielddouble instead of delivering properly interlaced material. (Probably just a pointer error, where so they copy the same field twice)
If you have registered I'm sure they will have a look at the problem and fix it.
Meanwhile, use:
AVISource("file.avi",pixel_type="yuy2")
converttoyv12(interlaced=true)
and the newest CVS binary.