I had the same problem ~2 years ago.  Intermittent failure, but
tapping the starter would make usually make it work again.

The problem turned out to be not the starter itself, but the
solenoid/control connection (the wire that goes to the ignition
switch) at the starter itself.  On my originally diesel van, this
connection was made by a 1/4" quick connect, so apologies if it's not
relevant to your setup.  There was some corrosion, but the original
wiring was also stiff with age, putting a lot of stress on that
connection.  I cut off the original quick-connect, spliced in a short
length of 12ga "ultra flex" wire with a new female quick connect to
minimize stress on the connection, cable tied the original wire to the
starter for strain relief, wire brushed the terminals, and hooked up
the new quick connect with some dielectric grease, and it's been good
ever since.

Good luck; hopefully your issue will be as cheap and straightforward to fix.

Alex