I found the error when processing the output from mythtranscode with avconv.
invalid non monotonically increasing dts to muxer in stream 1
I fixed it with a stream copy using mencoder
mencoder "input.mpg" -oac copy -ovc copy -o output.mp4
The same technique with avconv did not work, it crashed with the same non monotonic error.
avconv -y -i "input.mpg" -c:v copy -c:a copy output.mp4