Is the slideshow and the movie joined together, i.e. via iMovie or iPhoto? If they are two separate entities then the reason the slideshow stops or hesitates at 99 is that's the standard for items in a slideshow. Although iDVD 6 now allows more than 99 slides in a single slideshow the chapter marker numbering for each slide starts over at 1 after #99. I've burned a couple of slideshows with around 150 and get that pause/hesitation at #100.

I would suggest you burn at the slowest possible speed to obtain the cleanest burn possible. Until my new G5 I would burn at 1X (had to) and never had a coaster. I burned my current set at 4X and they seem quite compatible with all of the players we've tried so far. They will hesitate but should move on.