I pulled the solenoid and motor from the mounting plate.  All the gears are good with fresh looking grease.  I reassembled then removed the back plate from the solenoid.  All looked well but somewhat dirty.  I removed the solenoid spring and rod and noticed some "dirt".  Not really dirt but normal build up of residue from the moving parts.  So I cleaned it well with some carb cleaner.  The cover plate has a good rubber seal so normal road dirt won't get in.  The electro-magnet contacts looked a bit corroded so I cleaned those up too.  I reassembled the solenoid and test fired it on the bench.  It fired well and the motor turned well.  I reinstalled it and so far it sounds better and has not failed even after getting the car fully up to operating temp.  Time will tell.  It is a rebuilt Delco unit so I wanted to keep it.  The PO had it changed 8 years ago for similar symptoms.

 

It had no shims installed and the clearance on the gear was just a squeak tight, only about 1/64" of an inch too close so I put one layer of aluminum tape on the mounting surface.  It seems much better now.

 

So, if you are mechanically inclined enough to change the starter consider a little clean up instead of a replacement if you suspect that the solenoid is the bad part.  There is little that can go wrong inside the solenoid.  Even if one of the wires are broken they can be resoldered.

 

Steve