If it spins over like normal once it actually turns over then it is not the battery. More than likely a dead spot in the starter motor windings or a short in the solenoid.
If you hear a noticeable click or clunk when you hit the key and it doesn't spin over then probably dead spot in the motor windings. If no click (solenoid) then bad solenoid.
COuld easily be a poor ground as well. Without crawling all over the car, install a new ground wire, 8-10ga min from somewhere on the engine to the body. Also shorted + wire to starter or wires to solenoid could have the same intermittent issue.