Put a volt meter across the battery and get a friend to crank over the engine does the battery stay above 9.6 volts? Check that connections are clean tight and secure. An inductive amp meter would be needed to see what the starter is drawing. I should be approx 120 amps on initial starter engagement and drop to 80 amps after a second or so, higher amperage would indicate a bad starter if battery voltage to the starter is good. Look at youtube there are several tests on how to measure "voltage drop " in an electrical circuit.