The furance doesn't want to start up.You can hear the fan motor hum as if it is going to run but after a min.or so it shuts down.If I lower the thermostate all the way down and leave it for several hours it will then run.I have replaced the Thermostate

When you say fan motor-- Inducer motor or blower motor? If you turn the fan switch to on at the thermostat will the blower run? If no you have a blower motor or motor run capacitor that is bad.