My Air Conditioner/ Heater Blower will not stay on? on 1994 Ford Explorer

I have already installed a new blower and new switch. It runs just fine some times, but then when I have stop at a store and get back in it will not work. Then later for no reason at all it will start working again. Is there some kind of adjustment?

Asked by for the 1994 Ford Explorer
check the relay mine did same thing and it was the relay