I have a problem mentioned in this forum but did not see an answer. My Johnson sealed propane fireplace has a wall thermostat control. Ran out of propane in big tank so everything turned off; had filled; now fireplace delays starts for about a minute then ignites with big boom and blows itself out; scary when that happens. Pilot still works but sometimes blow out when boom happens. Gas tech said outside valve had variable pressures so changed it out with no change in problem. Funny thing is have same fireplace upstairs and it still functions perfecty. I can change a thermocouple if that is the problem; I think I can. Thanks for any help.