When heat is called for, the gas furnace will start the draft inducer fan, the spark ignitor with click away, the flames with start and then after 15 sec to one minute it will shut down. Then it will repeat and sometimes stay on until heat is no longer needed. I've seen it a few times shart and shut down for a while. Any suggestions for a fix? Also, after the flames start, the spark ignitor may continue clicking away for a while. Thanks.