Carlos is right it is called a "fan limit switch" it senses the air teperature coming off the heat exchanger, if you installed a larger or more powerful blower motor it is cooling the heat exchanger off too much and therefore the limit switch will shut it down until the temperature comes back up to where it needs to be and then will allow it to come back on. I would say that you have too much blower speed!!!!! also as Carlos said depending on the motor that was installed there should be a way to slow down the fan by changing the wires, most of the blower motors that are installed these days are what is called a "multi-tap motor" you can change speed or even voltage depending on which way it is wired. I am not an expert because I have only been in the HVAC business for 10 years :oops: But I would say that is what your problem is. Hope this helps, Later Lonnie