A/C and Heat quiets working every now and then.
Look under the passenger side of the dash behind the glove box. You will see the blower motor. Right next to it is the blower motor resistor ( connector with 4 wires going in; orange, light blue, tan, and yellow). Pull the connector off and check the contacts to see if they are burnt or loose.
The next time it stops blowing, pullover and with the fan set to high speed, reach under the passenger side of the dash, and with the handle of a screwdriver rap on the blower motor.
If the blower starts working, replace the blower motor.