Yeah, I've been trying to chase mine down--because the Prev owner got a new starter recently. I thought it had a correlation to battery voltage because I'd have it where it'd spin, spin, spin, then I'd switch over to the backup battery and it'd start. BUT, the other day I had it spin, spin, spin, start. So maybe not. I've read a lot of threads on it, and it seems to come down to either the Bendix (starter drive) or the possibility that there's worn teeth on the starter and/or flywheel.
I also noticed that my weaksauce stock alternator wasn't putting out the volts, even when the engine was revving good. Then the other day I noticed that the belt seemed a bit loose (like I could grab the fins of the alternator just behind the pully and slip it by hand. So I of course tightened up the belt. Hopefully that gets the alternator back on track. Of course, it's such a meager output that if I have to replace it and upgrade I won't cry (other than the obvious cost considerations).
It's always something, isn't it?!?!?!