Runs rough when hot

[CalG]

Hi, everyone, I've had a Ford 8n, has run well for years, I've accumulated about 150 hours on the present coil, points, plugs, condenser. I cleaned the points, set the ignition timing and tested, it was running great. After an hour of mowing, it lost power and was backfiring through the exhaust system. After it cooled down, I tested each cylinder, it was getting a strong spark, and it fired right up and ran without problem. The coil was bench tested and is within spec. I'm thinking either 1) the timing somehow changed, or 2) the coil may be breaking down from the heat of operation. I want to get some other opinions. Please feel free to opine or suggest additional tests. Thanks!

condenser and then coil,

but neither are costly.

It's not the timing, and if starts right up, it's not engine wear.

Fuel deliver? Maybe.

My money is on the ignition.

Bench testing a coil is about meaningless unless it has failed completely. But then it wouldn't start ;-)