'79 F-250 Ignition Problem

I have a '79 Ford F250 that I use primarily for plowing snow. It always starts up great, including now actually,however, but I have recently developed a problem of the engine dying after only 10-15 minutes of operation. When this occurs, I've checked for spark at the plugs and there is absolutely none. Zero. The truck will not start. If I try an hour or two later. Still nothing...nothing until a day later and the truck starts up easily on the first try. 10-15 minutes later, dead again. I always lose spark, but it always starts right up a day or several hours later. As always it will only run 10-15 minutes, then die. I first assumed the problem was the coil, so replaced it. Still the same problem exists. Next I replaced the ignition control module (brain box as some people call it) thinking this was the culprit. No. Didn't help. I'm thinking the only thing left to do is replace the distributor, but don't understand why this is happening only when the engine seems to be fully warmed up. Plug wires are good and everything visible inside the distributor looks fine. Appreciate any help or input I can get on this one.

Check the TFI Module, including wiggling the wires while engine is running after norm. operating temp. has been reached.

It could be the magnetic pickup assembly in the distributor goes bad once it gets warm and everything starts expanding from the heat, you might be able to get away with just replacing that part of the distributor.