engine not turning over 79 SC
 

The facts(?)
I went into a store for 15 minutes
When I came out the engine did not turn over.
The horn honked.
The battery seemed strong because with the ignition on
the radio/CD player worked.
The voltmeter showed over 12 volts.
I switched off the CD player and still did not turn over.
I let the car roll forward and let out the clutch in gear.
Twisted the key and the engine turned over and started.
The voltmeter shows 13+ volts while the car is running,
Went home (1 mile) Pulled it into the garage. Turned it on/off several times and it turned over and started easily.
This has happened before.
Could it be the ignition?
Or the starter?

You're describing the exact situation I was in right before my starter went out.

I'm thinking ignition switch first, starter second.

I should also add that when I turned the ignition there was no spring to it. So I'm thinking ignition switch. I have a spare, so tomorrow in it goes.

Something similar happened to me recently. Went on a long trip(mostly freeway with some rain), stopped for gas on the way back and the car refused to crank at all. Everthing electricly seemed Ok and the car started instantly with a short push. Got it home and it still would not crank. Came out later after the car had cooled and it cranked no problem at all. Its been fine ever since but now I don't quite trust the car.

I was thinking solenoid perhaps or starter itself

I'd suspect the starter/solenoid first because of the bump start scenario and the fact starters tend to fail more often than switches. Your comment about the spring in the switch, though, makes it a different situation and since you have a spare, replacement makes good sense. Just be prepared for a repeat of the start failure.

As these things age, and then get hot , they seem to want to stick. Sometimes hitting the starter w/ a hammer frees things up. They can also be taken apart and cleaned. When my starter began doing this w/ 175000 mi on it I decided to replace it w/ a new hi torgue model.