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.
Working in a customer service oriented job I can truly appreciate minute by minute notifications and incredible customer service. I am building my seventh fox and have dealt with every known distributor on the planet and LMR is by far the most efficient and thorough company I have dealt with.(twisted wedge top end kit and supporting equipment project starting this week)