Nick:
Thanks for the suggestions. It's looking like the problem is not specific to FS after all, but I'll recount the latest clues in case you hit this again.
1. NVidia version is 1.0-8765.
2. glxgears actually fails in same way as FS programs. Hmmm.
3. Color depth, as reported by GUI tools is supposedly "millions of colors" (presumably 24 bit).
Anyhow, looks like I need to pursue this as a generic GL problem.
Graham