Problem solved. The app uses the Visual FoxExpress 5.0 framework, which includes a lot of class libraries and programs.

I went through the project and individually ran COMPILE CLASSLIB on all the class libraries (both VFE and my own). To my surprise they all compiled OK. In the past I've had subtle corruption problems with class libraries and lost a signficant amount of work on a couple of occasions.

Next step was to individually COMPILE the VFE utility programs. These never change. BUT, one of them wouldn't compile - "line is too long". Somehow, a 5K procedure file (that never gets touched) containing a few public functions got itself bloated up to 28.7MB, padded with nulls (CHR(0)) on the end. Deleted the nulls, and all is well (albeit most of a day later).

And it's not like I'm running W95 on flaky AC power, either. My W/S is a solid W2KSP4 box on a UPS.

Cue the theme to "Twilight Zone" ...