I would say that you have a bad battery or a bad connection! A battery may show 12 volts but not have the amps to get the job done, a load test will show that. If the battery checks out bad then it needs too ne replaced. If the battery checks out good then you have a bad connection somewhere; and the problem is "where is somewhere"? Get a toothbrush size wire brush, a can of contact cleaner (or tuner cleaner), and a tube of dielectric grease. WHILE WORKING ON THE + CONNECTIONS THE BATTERY SHOULD NOT BE GROUNDED!!! Between the battery and the starter at each connection; take it apart, with the cleaner and brush clean both sides of the wire terminal and the contact to which it connects, smear it with grease and reassemble. Do all the connections from the battery to the starter; and do both ends of the battery ground wire.

As I said before if the battery is bad it may show volts but not have amps available, if the battery is good it has the amps but a lousey connection can prevent the amps from getting through.

Clifford