Just thought I'd toss this out.
My '77 chevy truck kept showing lower and lower voltage on the guage. I kept checking the battery and alternator and all seemed ok. Well, my water pump died so I put on a new one a few days back.
Guess what? My volt guage is back up again. The negative cable goes from the battery to the alt bracket. Bracket goes to the water pump and aluminum intake. Guess the bolts were crusty and not giving me a good ground.
Sometimes its the least likely, goofiest things that cause problems.