Update:
Burned the DVD5 NTSC version at 8x onto a Verbatim 16x DVD+R, booktype set to DVD-ROM. Exactly the same problem as the first disc in the same places. Used Nero on Windows this time.
Then burned the DVD9 PAL version onto an Imation DL, also DVD-ROM booktype. This one plays flawlessly (or at least in the spots I've checked it).
So - perhaps this was some weird PAL/NTSC problem, or perhaps two bad burns. Perhaps an unusual combination with my particular player and the DVD5 ISO.
Anyway, great stuff - now I can watch it flaw free!