If your starter develops a "flat" spot, you can use any 1/2" - 5/8" diameter rod about 24-30" long to "tap" it loose. A brass grounding rod available from Home Depot is readily available. You push the rod straight down at the back of the engine until it touches the starter area. Take a flashlight & shine down behind the engine from above and you should see the area that you want to tap. Give it a few sharp raps with a hammer to help the starter get off the "flat" spot and it should start ... if that is indeed the problem.
Thanks to Gordon Killebrew (I learned this at his school a few years back).
btw, if this does get you going, it's probably wise to start looking for a new starter or having the existing one rebuilt. Changing the starter is very easy ... after you've removed the plenum, coil packs, etc. etc.
George