Hello: You’re mixing up tidbits about gas and electric heaters. Gas heaters have only the thermostat at the bottom of the tank. The two main suspects are the thermocouple and the gas control. If the thermocouple works OK, than most likely it’s the gas control. Other things can play into it, like air flow into and out of the combustion chamber and how well the thermocouple is being heated and how dry everything in the combustion chamber is. Also, as its gas you’re playing with, it is a good idea to get help if needed to keep things safe. Intermittant trouble is always the most difficult to deal with. Do read up on gas heaters in Tanklets and good luck!
ps. Your problem may be as simple as the thermocouple not being heated enough. 😉