8N ford quits running

Another possibility, though much less likely, is electrical resistance causing spark failure. Also easily detected.

Sounds like yer "flooding" yer engine when you ask for more power and its taking about 5-mins for yer sparkies to dry out. Check your firing order; 1,2,4,3 CCW. These old Ford N-engines are so great, they'll start on 2-cylinders but won't have enny power. I'm gonnna assume you have the 4-nipple squarecan frontmount distributor. Thats:

Sparkie #1 is connected to nipple #1 at 10 o'clock
Sparkie #2 is connected to nipple #2 at 8 o'clock
Simple, eh? now PAY ATTENTION 'cuz it gitts tricky here
Sparkie #4 is connected to nipple #3 at 4 o'clock
Sparkie #3 is connected to nipple #4 at 2 o'clock
Therefore firing oreder is 1,2,4,3 CCW, isn't that amazing?

HTH, Dell, yer self-appointed sparkie-meister

There is a 1/8" pipe thread plug in the bottom of the carburetor's fuel bowl. Remove it, and watch the flow of gasoline. About 1/2 cup or so will rapidly run out, then the flow will drop off a bit, to what can flow though the carburetor's inlet valve. This flow must continue steadily at a level that is at leat what the engine needs under load.

If the flow tapers off to little or nothing, you have a restriction in the fuel system that needs to be cleaned to get an adequate flow of gasoline to the carburetor.

If the flow of gas is adequate, you may have a ignition coil or condensor that fails as it heats up, or even a bad ignition switch. Post back after you check the gas flow.