when I turn on my gas fireplace the flame will go across the entire burner. Then seconds later the flame creeps back till only a few inches of the burner is lit. As the fireplace heats up the flame goes back across the whole burner. If I turn off the heated fireplace and turn it back on the flame will go across the entire burner. The problem only happens when fireplace has not been used in some time and it’s cold. Any help would be greatly appreciated!