I have a Trane gas furnace with electric pilot. I'm guessing it's about 10 years old. My problem is intermittent. The normal sequence for heating goes ignitor, then burner, then burner heats flame rod, then blower. Sometimes, mine skips right past burner sequence and goes straight to blower, thereby blowing cold air. Not the flame rod, I just replace that. What could be the other possible causes??