You know, this probably isn't what's causing your problem, but ya never know:
One time my wife's minivan kept having problems...clickity-clack sound when trying to start...then later it would start right up....then same thing again.
I kept cleaning the batt terminals, checking wires, etc etc...used the volt meter and it would show 12 volts +...check it again and just a few volts would show. She took it to Pep Boys, they checked battery, and said nothing was wrong with it.....then she tries to start it up...clickity-clack again. They were closing up but they came out to help. Finally, they just put in a new battery and everywhere has worked right since! Just a wacky battery. The best thing is they gave her the battery for free!