Insert key into Ignition Switch
Turn Key
Engine cranks to start engine
First try always cranks engine. In 29.5 years it never failed to crank.
If car sets 2 or more days:
Turn Key
Sometimes Engine starts and immediately dies. Turn Key to re-start.
a: Hear a click like a Relay or Solenoid
b: All lights on dash dim as long as key is held.
c: After many tries it will crank and start.
d: I have pushed the car in 5th gear causing the Fly Wheel to move. Sometimes
helps but not always.
Once car starts, you can start and stop all day long and it never fails to crank/start.

 

I would bet a combination of several things all ganging up on your starting system.

Starter solenoid in need of new contacts and plunger

Battery cables terminals and battery getting old and tired.

B terminal of the alternator wire to the fuse block thermal cycled so the charging system is not working as good as it once did.

Some slight drain on the battery .

 

Also try cleaning the contacts in the Starter Relay. Battery could start to be getting old and weak. If driven daily the battery may have enough juice to fire it up the next day but sitting it doesn't get recharged to where it may need to be.

When you are rocking it in 5th gear, you may be getting it off of a compression stroke taking the strain off of the starter, in turn taking the strain off of the battery.