I have a Coleman gas furnace about 7 years old, has an electric ignition. The fan starts up and it attempts to ignite but wont saty on. After 3rd try it shuts off completely. I have had the igniter recently replaced as well as the venting motor. The furnace worked fine for a while but after a month the same problem came back. What could it be? The gas company has seemed to misdiagnose twice already, I'm weary to call them again.