Your Mustang's manifold absolute pressure sensor or MAP sensor is located behind the firewall and provides manifold vacuum readings to the PCM. Make sure yours is in perfect working order with a replacement sensor from Latemodel Restoration Supply.
I have been ordering items from Latemodel Restoration for many years. I've found them to be exceptionally professional, timely with order processing and shipping correct items ordered. I've never received an incorrect item on an order.