1979 International Scout starter dragging

When I attempt to start the starter drags very slowly, not nearly fast enough to start the engine. It acts as if the battery is low but have continually checked and battery is at 12.5-13 volts. I replaced the starter, plugs and wires. I am showing the same volts at the solenoid as at the battery within.2. In many attempts I have gotten it to start twice and both times it was in the midst of trying over and over that it would all of a sudden crank faster and start up