chapter markers off in burned DVD

[Dan Christensen]

I just made a DVD using iMovie '06 and iDVD '08 version 7.0 (1068) under
Mac OS X 10.4.10.  I put in chapter markers using iMovie, and in the
scene selection menu in the iDVD preview the scenes link to the right
spots.  But after burning to a DVD, the chapter markers are off a little
bit.  They are correct near the beginning but get gradually worse
towards the end of the DVD until they are about 4 or 5 seconds off.  I
verified this by playing the DVD in a standalone Sony player and also by
playing it in my Mac using DVD Player.  The problem shows up both when
using the scene selection menu and when using the |< and >| buttons to
move between chapters.

The source of the clip is DV from an inexpensive camcorder.  The DVD
is 124 minutes long, with 28 chapter markers.  I chose Professional
Quality for the encoding.

Any ideas what could cause this?

Thanks,

Dan