1993 GMC C1500 Occasional Starter Grinding

The starter on my truck grinds occasionally, sounds like it's not engaging the flywheel or grinding against the flywheel. I've replaced the starter and had it looked over by a starter shop. Replaced the flywheel and inserted shims but it still tends to grind ever once in a while but then its okay for a while. I've asked different mechanics, the guys at the local NAPA, and friends but nobody seems to know how to fix it. Would the starter bolts not being tight enough cause this? If so, what are the torque specs for the starter bolts? Thanks for any advice.