Neptune mdg3000aww starts heating and runs fine but stops heating shortly after start up can restart heater starts again and shuts down

Sounds like the motor is going into thermal overload and shutting down. If you here a Buzzing noise when you start dryer motors bad. If after you start the dryer wait and stop it after 10 min and then try and restart it if it buzzes and rattles bad motor.

Yes, I recommend you type in "Sears Parts Direct" to your computer search engine.
Put in your model number in the box, and you will see schematics that will answer your questions.
God bless your efforts.

Does this only happen when the engine is warm or at operating temperature or all the time even when cold? In older engines, a 'bad ignition coil' would fail when the engine warmed up and would not start again until cool or at ambient temperature.

LOOK FOR BLOWN WINDOW CIRCUIT BREAKER.YOU HAVE A SHORT IN WINDOW SWITCH OR BAD WINDOW ELECTRIC MOTOR.YOUR HEAT PROBLEM IS A STOPPED UP HEATER CORES.THE STOP LEAK STOP ITS HEAT.THE STOP LEAK HAS STOPPED OR PLUGGED YOUR HEATER CORE SO COOLANT CANT PASS THROUGH IT.YOU GOING HAVE TO FIX RADIATOR LEAK AND REPLACE HEATER CORE TO GET HEAT AGAIN.

the gas orifices may be plugged try to poke them out with a fine wire . shyt off power and gas before cleaning as you don't want to be burned

Strange. Heaters usually shut down with the spinning drum when door is open. If you check the schematic for unit you can check to see if in fact this is correct operation. Attached site usually has schematics for appliances. You might want to check. Search Over 2 1 Million Appliance Parts

check and see if you get water/coolant circulates when you have the cap off (only check from cold start) the original water pump came with a plastic impeller and tends to strip on the shaft also check your thermostat before replacing water pump.

possibly your heater is shorting.

sounds as if the thermal protector is shutting down the motor due to overheating. time for a new motor

you check all the wirings maybe you have lost contact wirings!