On a furnace that old, the thermocouple is likely not electronic. It is likely that it is a capillary tube type. In my experience they either work or don't work. They don't work intermittently. I would suspect a weak pilot flame or a back draft blowing out the flame. If a building is tightly sealed, just opening the door can cause enough back draft to blow out the pilot light if the stack is the only opening into the room. The same is true of a leaky building. Wind gusts can blow out the pilot light. A missing baffle can also cause the problem, you know that little sheet metal piece that you have to remove to relight it that you didn't bother to put back in place because it was a pain to get out and a pain to put back.