start-time goes to arbitrary places

[tmoran]

Sometimes the value of start-time is roughly tripled, eg "start-time 3" starts 10 seconds in, and on other video files it's total nonsense, ie, 0 goes to the start, 1 goes several minutes in , 2 is somewhere else entirely, etc.
What's going on?
Is there a reliable way to start playing a certain length of time into the video file (eg, to skip a black or bars&tone leadin?
VLC 0.8.6d Janus