I moved the set of audio clips to track 2 and it works fine there. It's only on track 1 where it fails to play after 18 minutes.
So it has nothing to do with my system. It's set up for performance use and has more than enough resources.
I created a new project and copied the clips to it and had the same trouble so there's something screwy with track 1 in X2, it seems.
Normally I kept each clip on it's own track and record them to tape individually. Putting them all on one track was meant to speed up the recording process.
I'm not going to try exporting the project or mixing it down to see if that works because I have no need for that in this case.
Next time I'll just put them all on track 2 and file a bug report, I guess.
Thanks for the ideas,
Steve