sporadic starts for me were due to gunk in the pilot hole. There's a tiny hole in the back of the burner that can get a speck of ash in it - that can cause sporadic starts. Also if the 'fleece' is super warm that can also screw it up. Webasto says you can't replace the fleece - it's a permanent part of the burner (which you can replace). The fleece is super well attached, and seems to play a critical part in how it runs (they did call it some kind of ferro combustion). If in doubt, call the Webasto technical service. Amazingly helpful, I've called them many-a-time.