My Garmin 800 has not been showing turn-by-turn directions consistently during the last several rides. Friends using the same course maps on the same model Garmin don't seem to have this problem. I've tried re-booting the course, with indifferent results.

All advice will be much appreciated; I'm getting too damned old to squint at street signs!

I have had that problem before. There are different map formats and I didn't download the right one. Just a though.

Could be firmware versions, device needing a reboot, differing maps as mentioned above. Suggest going to Garmin's official user's forum: https://forums.garmin.com/forumdisplay.php?245-Edge-800