Starter grounding internally

First off. 96, 4.0, ax-15
A few weeks ago my jeep wouldnt start.. No symptoms of bad battery. It would have power when the key was turned to accessory but when going to start it, the power would die and not come back. Figured it was grounding out somewhere. Went over all the power and ground wires and they were good.
So my dad suggested the starter was shorted out internally. Went out and got another one and she started right up.
Tonite I was doing work and the same symptoms popped up. I'm going to take the starter and go get it warrantied but was this just a freak occurrence or is this caused by something else?
Thanks

So update to this. It's damn cold out and I didn't feel like replacing the starter so I unhooked the positive battery cable and rehooked it back up. Started right up with no problem.
Also decided to bench test the old starter. First few tries on the bench a click and nothing. After a few tries with it, the starter would spin. This pretty much went back and forth.
Any ideas?

Try smacking the starter with a hammer. That usually gets mine going again, but once it requires a hammer to get it working, you're on borrowed time.

It sounds like you have two issues, a crummy starter / crummy replacement starters and possibly a bad wire or connection at the battery.

I'm going to keep the starter that I just replaced in there for now. I'm also going to wind up ordering a set of 5-90's cables and just replace everything. While everything seems good it can't hurt to upgrade.

clean your battery terminals, sand them till they are shiny

1. Start with testing the battery. Fully charge it and then have it load tested with a carbon-pile tester; battery should hold 1/2 of its CCA value for 15 seconds without dropping below 10 volts.

2. If the battery tests good, then clean both ends of both battery cables and then do a voltage drop test--google it.

3. If the battery tests good and the cables test good, then make sure the ignition is sending a good signal to the starter solenoid. If that checks good you likely have a starter problem.

You jumped ahead on the diagnostics tree. When you do that you end up chasing your tail.