I seem to have an erratic problem with dvdauthor

On Some DVDs, at the point of the start of the original vob file the DVD stutters and repeats (around 10-15 seconds) and wont move past it
Going to next chapter works and rew to just after it file plays fine, but wont play or fwd though it.

I normally combine at least three - four vobs per DVD and so far have only seen this happen once in any particular DVD, sometimes at start of DVD sometimes later (but always at start of original file)

Anyone any ideas where problem could be.

Creation process is
Video to hard disk using bttv/ffmpeg
Avidemux to chop out stuff I dont want
ffmpeg to transcode to vob
dvdauthor with no menus but with chapters
growisofs

The original vobs play fine in ffplay/vlc

any help appreciated