I was having troubles keeping my Lopi berkshire gas stove going. It would burn for 15 minutes then go out. I'd relight the pilot light and everything would check out but it would do the same 15m burn. I changed the thermopile and thermocouple still same 15m burn. I decided to check my chimney it is a duralast conversion 4" aluminum flex and it had about 2" buildup of soot. Stove has been used for about 6yrs. Is cleaning the chimney needed on a gas stove? Is something misadjusted on my stove? Thanks