Hmmm, that doesn't sound great. It's kind of weird as I've run another test with a fairly long file that was recorded recently and it worked perfectly well. Run time was about 40 minutes.
The file giving me trouble was recorded by someone else about 12 months ago on a different camera, an older GoPro model. I've burned DVDs of the material previously using basic burner software (Burn, free software that was quite capable) but had to upgrade to Toast as older software is gradually stopping working, presumably because of the switch to 64-bit only on the Mac. It's frustrating but the problem does seem to be related to one particular file. I'll try splitting it and see what happens as it has a long runtime of well over an hour.