It sounds (pun) like the gear is not engaging the flywheel completely. There may be teeth missing on the flexplate. The only way to be sure about the teeth is to remove t he starter and turn the engine by hand while examining the teeth on the flexplate. A dab of paint on the flexplate will insure you inspect all the teeth because you can stop when you see the dab of paint come back around again.
You do have 3 bolts holding the starter to the bellhousing, right?