I've got a sample here which I think shows the problem (warning 52MB file)
As the two tennis players cross the court the picture starts to show major artefacts under VDPAU and MythTV.
I've tested this under MythTV with the ffmpeg code without issue, and recent builds of mplayer don't have any issue with either VDPAU or ffmpeg based playback.
Under MythTV i've tried all of the various VDPAU interlacer options, including disabling the de-interlacer.
The video sample is an off air freeview|HD capture in 720p from NZ with H.264 video and HE-AACv2 audio.