I just had the same dam thing happen to me this last weekend. I have a RobbMc mini starter. My theory is its not shimed correctly and the solinoid is stuck halfway between being engaged and fully retracted.

I have seen this under my car while working on it. The gear being hung up on the flywheel.

Check under the car next time this happens. If you see the starter gear on the flywheel. Give if a hit and try the ignition again.