For years my (24 yr old) Ruud gas pack burner and fan have shut down at cycle end, simultaneously, with a click at the thermostat. Recently, though, the burner will shut down (with tstat click) but the fan will continue to blow for 3-5 minutes. The air runs cool then eventually shuts down, leaving a chilling feeling behind. I'm certain this is shortening the cycling time and costing me money! Any thoughts about replacing or adjusting fan control?