I had this exact problem once. There is the module on the side of the distributor. It was actually fine.
The part inside the distributor that it plugs into went bad. Tells the computer when to fire the injectors. You have to take your distributor apart and replace that module. They should sell one at the parts store.
1990 Mustang GT
Dart IE 363, PT78 HPS, Built 4R70W, BS3, BBRC Fuel system, TeamZ, ect.....
Tuned by Bob Kurgan