I had a magnetic pickup do this in a '75 Powerwagon a few years back. It would run fine until it got warmed up and then wouldn't start. Turned over fine, though, just no spark.
I've had them do that too.
Just remember....air, fuel, spark, compression. That's all it needs.
The odds of it losing compression and air are slim.
It's most likely losing fuel or spark.
Check for fuel spark with the condition duplicated.
Quick test for ballast resistor equipped stuff, multimeter required:
Check coil + voltage key in start vs. key in run. Should have battery voltage in "start", should be around 9v or slightly less depending on ballast value with key in "run". If you've got less than battery voltage with the key in "start" then you're probably on the right track. (This is a pretty common wiring error).
