The starter rebuild guy called me back today. He told me the exact same thing. He said he is seeing so many solenoids getting lower voltage due to cost cutting on wire sizes that he sells a kit with a relay and prewired socket to add onto the solenoid circuit. He thinks the solenoid may have been stuck on a bad spot from low voltage and my gentle tap "fixed" the solenoid. I am going to add the relay and see if it happens again. If it happens again after that he will fix my starter under warranty.
It will be interesting to see if this corrects it. This is commonly done on large equipment when you have long runs of 15 or 20 feet from the key switch. This is a short run and not usually an issue. However a quick verification if the starter is accessible is to use a jumper wire from the battery positive cable on the starter and the solenoid input or signal wire. If it snaps in it's a voltage drop issue. Of course if it doesn't the starter is at fault.
This reminds me of my 1957 VW bus. The starter/solenoid set up had three terminals: one side went to ground, the other side went to the positive terminal on the battery, the third was the wire from the key switch on the dash.However a quick verification if the starter is accessible is to use a jumper wire from the battery positive cable on the starter and the solenoid input or signal wire. If it snaps in it's a voltage drop issue. Of course if it doesn't the starter is at fault.
After rebuilding my engine a piece of sheet metal was omitted (oops my error) between the engine and the transmission. This allowed access to the starter/solenoid terminals. With a stubby screw driver it was possible to short the battery to the key switch terminal and turn the engine over to start. This was eventually fixed with the little relay made for the purpose.
jtk
"A pessimist sees the difficulty in every opportunity; an optimist sees the opportunity in every difficulty."
- Sir Winston Churchill (1874-1965)
I didn't think about trying a wire from the battery directly to the starter. I'll try it if/when it happens again.