Most likely some thing or someone has gotten into the burner tube and changed the air ratio. Spiders like to build nests in there. Stink bugs hold family meeting in there.
remove the tube and inspect. Try not to move the adjustment slide when you do this. This assumes the flame was right before the problem and once the tube is clean and put back on things should be fine.
WBCCI 12156 AIR 3144 WACHUNG TAC NJ6
2004 Excursion 4x4
1991 34 ft. Excella +220,000 miles, new laminated flooring, new upholstery, new 3200 lbs axles