It sounds like the fuel in the fount is getting too hot and the heated vapors are igniting as they escape from around the base of the burner or some other opening near the flame. Heat is transmitted from the burner down the wick and into the fount, that's supposed to happen, it's part of the process. I would suggest you make sure the fount is not over filled and that there are no pin holes in the top of the air chamber. That you're using fuel of the correct flash point, there have been warnings about Klean-strip brand actually being paint thinner labeled as kerosene and too dangerous to use in lanterns and lamps. you might also check that your air tubes are clear of any obstructions like spider webs and maybe you should not turn the wick down so low, but I've burned lanterns for hours with the wick turned down even with the burner tube with no ill effects. Kerosene will ignite when vaporized even without a wick as long as it can find a source of ignition so be careful. I always have fire extinguishers handy here
Willis