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?

1 answer
check the relay mine did same thing and it was the relay