My dryer gets warm, but not hot. The burner runs for 30 - 90 seconds, then turns off. After a few minutes, it re-ignites and runs for a similar amount of time. The fan and exhaust duct are clean. I have replaced: the thermostat on the tube that surrounds the burner; the thermostat/moisture sensor and the fuse on the exhaust vent (just after the fan), and the gas coils. I can't find any problems with the ignitor - no cracks or anything. It is 5 years old. What is going on here, how do I fix it?