It could be your air mix tube is dirty or the metal chimney that directs heat around the boiler is blocked with soot. That could explain the fact that the LP won't stay lit. Also, if the chimney is blocked or not allowing air to vent, it could be causing the electric element to shut down when heat builds up.
Bob & Donna
'98 Gulf Stream Sun Voyager DP being pushed by a '00 Beetle TDI