Gregg

I had this problem and after pulling the new (rebuilt) starter that I
installed and taking it back to be tested at the shop where I purchased it
the starter was working fine.  The owner of the shop (European Auto Electric
in Chicago, IL) explained to me that there is a bushing that the pin on the
end of starter drive gear fits into when the starter is engaged.  Over time
this bushing can go out of round and cause the starter to jam.  The problem
is worse at first when the motor is hot because of expansion but as the
bushing gets worse it jams when cold also.  I replaced the bushing a PITA
because you have to get it out without making the hole out of round.  The
starter worked fine.

I would check the starter first and if its good that may be your problem.
This was on an air-cooled motor don't know if water-cooled are the same.

Blitz