Snee,
This is not a joke.
Vibration and 40 years of getting bounced around can loosen things.
Honestly, I have had similar problems that only took finding and tightening all the connections all along the line.
And there is another problem that I have repaired for others but not had myself. Most of the AC wiring was done with solid wire romex and the vibration of the vehicle has actually caused it to fatigue and break. This usually happen very near a terminal. Sometimes under the insulation, so pull on the wires to check.
Matt
