Have you tried replacing the battery?  It could be a bad cell.  Or your
alternator could not be putting out enough voltage (should be around 14v).
Also you could have corrosion up inside the wires that run from the battery to
the starter.  Sometimes they corrode inside the shielding.  I guess the only
way to check on this is to cut some of the shielding back and inspect the wire
itself.  Ron sells the new wires I believe (they are over 9 feet long).
Have fun.
Ken Wilford
John 3:16