Hello, I have a Heatilator (about 10 years old but appears to be in good condition) with electronic ignition (no pilot) which will start sometimes and run for a few minutes, then shut down. Other times it will only start for a few seconds and shut down. In the best situation, I have only been able to get it to run for a maximum of 20 minutes before it shuts down. Service people checked and cleaned all wire contacts but still no success. Any ideas appreciated. Thanks.