Mine did something similar and I found a huge dirt dauber nest on the burner. Cleaned it and it works great. I have a screen over the exhaust so I don't know how they got in the tube.
97 Dodge 2500 Ext Cab 4X4 CTD with Goerend Transmission mod.(GVWR 8800lbs, or 4000 Kg)
95 Four Winds Fifth Wheel
99 Toyota Fourrunner V6