Why is this Avisynth script giving me wrong ordered fields?

Deinterlacing and going from DV to HuffYuv with this script

SetMemoryMax(768)
SetMTMode(3, 4)
FFMpegSource2("sourcevideo.avi", atrack=-1)
ConverttoYv12(interlaced=true)
SetMtMode(2)
QTGMC (Preset="fast")
RemoveGrain (mode=17)
crop (8,0,-8,-8)

Seems to result in backwards fields - i.e. when you view frame by frame with Virtualdub you get the forward/back motion.

Any suggestions?

Thanks.