ignition coil ?

My guess is the fuel in the line from fp to carb has vaporized

My stock 273 2bbl does almost the same thing. When the engine is at operating temp and at idle, the fuel filter will empty, the line pressure will stay at about 5psi, then the engine will start to decrease in rpm right before the pressure in the fuel line starts to drop, then the fuel filter refills, the pressure comes backup, the rpm increases, and then the cycle starts over again