sounds like the bendix. mine would do that once in a while. had to take it out and replace the bendix drive, thats the part that makes the starter drive jump out and engage the clutch gear to turn it over. they sell the drives on ebay. i have a open bdl belt drive so it wasnt that bad to do but you have a enclosed so your going to be taking the primary off. its a pain but just think when you fix it, youll be good to go for a long time.

 

Sounds like you need a new battery.
You might have 12.5volts..... but not enough umpgh
(my tech term) to start. I've had several batteries that
had plenty of volts.. but when the dealer hooked up to
their batt. checker they did not have the cranking POWER.