[HO455]

It sounds as if the starter is not engaging properly every time. That could cause it to make a bunch of noise one time as the starter gear can't mesh properly with the flywheel. Then the next time you try it catches the flywheel gear properly and starts. A common cause is an improperly shimmed starter. I have had GM starters that have needed as many as three shims to engage properly. A bad bushing in the Bendix can cause this too. A rebuilt/refurbished starter only has the most obviously bad components replaced, so the Bendix components could be good enough to pass the rebuilders test, but too worn to allow proper engagement of the gears when installed. The post by snj8198 is a good one as the incorrect bolts will not keep the starter from moving around. The torque of the bolts hold the starter down to the block but generally won't keep the starter from moving sideways. The knurled part acts like an index pin keeping the starter stable. V-8 engines will stop rotating most of the time in one of 3 or 4 spots thus the teeth on the flywheel don't wear evenly. Rotate the crankshaft by hand and inspect all of teeth. The gears will usually work OK with 20% of the material in a tooth missing. Several teeth next to each other missing 50% of the tooth material can be problematic. Hope this helps you resolve your issue.
Good luck.