I have a vangaurd heater natural gas , older model vn2550ITB , everytime i get it lit, it shuts down a couple mins later , it seems to act like it isnt getting enough gas to keep the grids lit, i changed the, .thermal cupler, and it still isnt working right ..do you think you can help me figure out what the heck could be wrong with this thing , the weather is cooling off and i like to use this heater , ..

Heating & Cooling - Vanguard - International VN30B Infra-Red Heater