I have a Intertherm thermastat in my mobile home that stops turning the blower on from time to time in the auto position, if I turn it to on position it works the blower fine, after that I can switch back to auto and it works for a while

Hi harley. The thermostat does not directly control the fan when in the auto position. In the auto position, the stat only sends a call for heat to the furnace, which then starts the heat cycle and the blower is kicked on through a relay in the furnace. So it sounds like your fan control in the furnace itself may be fualty. I you have a control board vs separate controls and relays, then you may need a complete new control board. However, since the fan is working well when the stat is in the on position, you can just let the fan run continuously, as long as the heat is working corectly when the stat calls for heat. Good luck, and please let me know what you find.