Starter not spinning

I just finished the motor swap on one of my mustangs, and the thing is, I wanted to just spin it over a few times, make sure nothing was leaking or breaking yet. I spun it over with plugs 1,2 and 3 disconnected. After that, I put the wires back on, and spun it over to start. To my surprise it actually started. Now it wasn't running perfectly, but it was running. I let it run for 5 seconds before shutting it down. I did another quick check for leaks, and broken things, none were found. So I get back in, and turn the key, nothing happens. The fuel pump came on, when I moved the key to the "start" posistion, the starter didn't come on like it did ealier. The solinoid is good, it was clicking with current on. I'm getting 12 volts to the starter, but its not spinning. After a few minutes of thinking, I tried again, and it spun. The after that, its not spinning again.

To me it sounds like the starter is bad, but I don't think this is really the case, The starter is pretty new.

Any ideas of what it might be?

I gave the starter a few love taps and that didn't help.

I'll let the battery charge some more, but It was at 11.2 Volts, so I left it on the 2 amp trickle charge for 24 hours, volt meter said 12.6 when I put the battery back in. I used the test light for the starter, that is why I said I'm getting 12 volts to the starter. I'll try to charge the batter up next weekend.

P.S. My red top held a charge pretty well, over 2 years since it was used, still read 11.2. Not bad me thinks....