First thing to try is replace the thermocouple if it has one. If it's dying it acts like what you describe. Also this is the simplest and cheapest thing it might be.
You're close, but H.E. furnaces dont have thermocouples, which require a constant pilot light. Studhauler needs to clean off his flame sensor, as scale will build up over time and impede its ability to tell the gas valve "steady flame, go ahead and open the main valve"First thing to try is replace the thermocouple if it has one. If it's dying it acts like what you describe. Also this is the simplest and cheapest thing it might be.
You have let the level of LP in the tank get too low. Don't let it drop below 30% full. You need vapor, not solid liquid. Re-fill when 40% low in really cold weather.When it gets down into the teens my stove and furnace both act-up..so i put a 60 watt
light bulb under the valve cover on the lp tank...Everything works fine then...Ferril gas
dude got ticked off...said not to do that any more...wrote me up on the ticket...So
after he left I put it back...