Starter Failure?

My 99 lumina has been experiencing wierd starts for 2 weeks now and has finally given up. The symptome had been a groaning, slow spin from the starter, but engine would fire right away. Now all I get is a click. Earlier on with this problem, I took the car to AAP and had the battery/alternator checked, and much to my surprise both checked out great. I attempted to jumpstart, but to no avail. It appears that my starter is gone, but is it? The solenoid clicks, I have plenty of amperage from the battery, comfirmed many different ways, but the starter won't engage or turn. Is there anything else that could cause this, or should I run to AAP and grab a new starter Mon. morning? Any help please.

it could be one of the three coils that the spark plug whires come from or your timing could be off hope this helps

If the battery and alternator both check out, then you need a starter.

Thanks to all who posted! As it turns out, it was the starter. I pulled it and took it to AAP to have it tested. It actually sparked tremendously and shocked the piss out of the clerk. I installed a new one and it fired right up. ;)