I've had the same problem off and on for 2 winters now. I don't really know anything about central heat units. My problem is that black sut is building up in the burning chamber. The flames are kind of tall I guess, and the tips of the flames are actually red. The pilot light continually goes out and relights within a few seconds. It does this 10 times on occasions within a couple of minutes. When I take the panal of the side of the unit there is all kinds of sut inside there. It will actually go through spurts where after I clean the unit by smacking the rails with a socket wrench (causing sut to fall out) it might actually run for a month without any problems. What could the problem be? The heater runs of LP gas.

If you are getting soot, your have too much fuel for the air. Older units had sort of an hour glass cut out with a damper you could adjust. If you see anything like that, try opening up the damper until you get a nice blue flame.

Does the furnace have a good source of combustion air? My 85,000 BTU gas furnace specifies 42 square inches free area to outside air, or 100 from a large house. In a tight house, kitchen and bath exhaust fans may rob the furnace of the air it needs.

Does it have forced draft, and is so, it is set up and working right?

Are you sure it has the right jets for LP gas? Many furnaces can run either natural gas or LP, but must have the right jets. Perhaps your LP supplier could help with that.

A lack of air creates more carbon monoxide and uses more fuel. You want to fix this problem. Call in a good furnace man if you can't find the problem yourself. You may want to contact the manufacturer for an installation and start up guide.