I have a 10-12 year old Intertherm gas furnace (split unit with AC outside) in my mobile home. Today it suddenly stopped blowing warm air. The thermostat clicks when it's turned up, the furnace kicks in and hums like it's lighting the burner, then no blower action to push the air through the ducts. Then the furnace kicks off after about 90 seconds of no fan. If I turn the thermostat switch to the middle "off" position between heat and cool and turn the fan from "auto" to "ventilation" the fan will run, just not when the heat is actually on. The furnace has a Robertshaw gas valve in it. I got out the troubleshooting manual and re-cycled the furnace (turn off gas and power, turn thermostat to minimum and "off", wait 10 minutes, turn gas and power back on, turn thermostat to "heat", turn temperature back up) to no avail. Filter is clean, there's gas getting to the kitchen stove right next to the furnace, can't figure out what's wrong. Thanks for any help you can give me.