I've now looked at the vob files with vlc and they all appear to be the same -- all stamped with 59.94 as a frame rate.
So I'm mystified by this. Have ripped more DVDs now, some come in correct, some don't.
If I use handbrake and convert each chapter separately, I can get them all to play properly at 23.976
So maybe this is a bug in the way Adobe interprets the frame rate of the vob file.
Thanks, John. I read that earlier today but there's not much specific to my situation.
But there's clearly something different about that first vob file that's confusing PP. Too bad -- because otherwise, PP is well and away the slickest way to handle this stuff.