You need to shim one or both mounting bolts away from the flywheel. This is a common problem with GM starters. Many times the rebuilder will machine the mounting face of the starter which brings it closer to the flywheel. This alters the correct gear mesh relationship of the starter to flywheel resulting in a binding or loose contact. You can buy starter shims at most parts places or just use washers. Usually one 1 mm thick washer under the bolt closest to the flywheel will suffice. But you might have to try several different combinations to get the correct one. Hope this helps.
