Before you spend any money vaccum the whole thing out,strip down what you can and vaccum all the holes and jets you can get to,use a small paintbrush to loosen crud on surfaces,be thorough make sure all holes are clear.When the fire is on and drawing air in to mix with the gas (so it will burn) it is also drawing dust,dog/cat hairs in and blocking up the jets (I don't care how clean you think your house is)if you call someone in this is the first thing they will do and if it solves the problem 1'st time easy money.No cost,no tech knowhow,just thoroughness,care and a bit of time.Happy New Year and recession.
Rich
December 2011