When I joined with Yoshimo his strength=17. When I crossed a wooden bridge I quicksaved and a couple of minutes later I noticed his strength now equals 1. Is this a glitch or does something in the game trigger this event? If it's a glitch, is there an easy repair with a character editor or ini file? Or do I simply start WAAAYYYYY back before I picked him up?

Can't say that's ever happened to me. Sounds like editing time.

Some undead/shadows will drain a character's strength temporarily. Check his stats, if it says drained then thats it, otherwise it's a bug.