/// These years were bears for air leaks. Use a spray can of carb cleaner to spray around the intake manifold while the engine is running. The worst case is that it is on the bottom of the manifold head joint (hard to get to). The fact that it runs until it heats up is the reverse of my experience for air leaks. They normally run poorly until the parts heat up and expand.

Failure on heat up points to a possible electrical problem. I mean you may have more than one problem. If you do not immediately find the air leak---move to eliminate a resistance caused failure in the primary electrical circuit. That is the battery, either or both battery clamps, and the red positive system wire. In my 73 it goes to a distribution block behind the battery. The problem, however, will be in the cables amd their end clamps. In really high resistance you can feel the heat generated if you immediately go to the battery on failure.

There can be a resistance caused problem in any plug in the system.