Is it the original gm style gear reduction starter? And if so are they the original starter bolts? When GM went to the gear reducted starter they changed the bolt hole size in the starter from 3/8 to 10mm. The block threads are still 3/8, so if a standard 3/8 starter bolt is used the starter can walk. The correct bolts for this starter can be bought by using starter bolts from a 99 Tahoe with a 5.7 liter. They will have the 10mm shoulder and 3/8 thread, and the starter will not walk with them.

Also, have you looked at the flywheel to make sure there isn't any damage to the teeth? Sounds like a bad spot in the flywheel is a possibility also.

Valid point....thanks Matt!