This can't be a new problem, as some old guy walking by the car the other day told me about it while I was trying to get it started.

I have a 39 Ford with it's original Flat head. the car starts and runs great, until it heats up, then I lose spark. Is this coil failure? and what does everybody else do about it?

the coil would be suspect in this case but if you still have the original ignition setup ie points and condensor i would be more suspicious of the cond breaking down after heating up also you may want to keep a test light or voltmeter handy to check for full voltage going into the coil when this happens good luck and hope this helps

yep. sounds right. either the coil or a condensor can do that. simple cure...replace with new.

kevin