Check your wires going to the starter! I know it sounds weird but on my Volvo Big truck the other year I went thru three alts and 4 sets of batteries before they found the real problem. A bad connection on the starter, so the company spent a few grand to fix what ended up being a freaking bad connection.
That truck's volt gauge was going from 9 volts up to 16 volts. It had no rhyme or reason for the way it did it.