Here is another thought.
This is a quote from Don...
"There is at least one other item that behaves in this manner (fails from heat and cures itself when it cools), which is a faulty connection in the primary ignition circuit (some electricians call these faulty connections cold joints, I suspect because they only work when they're cold).
Please skip this paragraph if this is more than you wanted to know, but I am told by normally reliable experts that an electrical circuit sees a weak connection on the verge of failure as a localized zone of high resistance which translates into a small amount of heat build-up.
As the heat builds up, the expansion created by the heat eventually causes the connection to open up (minuscule though this heat and expansion is, remember that a connection in this condition is on the virtual edge of failing anyway, even without the heat and expansion).
To keep things simple, before chasing the entire primary circuit which runs from the large battery terminal on the starter solenoid, through the ignition switch and back to the positive terminal of the coil,
you could simply connect a jumper wire from the large battery terminal on the starter solenoid to the positive terminal of the coil.
If the engine never shuts down as long as that jumper wire is installed, you will have confirmed that the primary ignition circuit is a cold joint in it someplace.
If the shutdowns continue with the jumper wire installed, you can check the remaining short section of the primary circuit which is between the negative terminal of the coil and the points.
Sometimes the conductor in this short wire develops a cold joint where it passes under the distributor cap or at the terminal of the coil or at the points themselves."
IMPORTANT NOTE: Installing the jumper wire between the starter solenoid and the coil is the very same thing as turning on the ignition switch, so you cannot leave that wire installed except when running the engine or you willburn out the coil or kill the batteries.
You can read the entire post and thread
here...