1991 5.0 Mustang gt E.C.U.?

I have a 1991 Mustang gt 5.0. About a month ago it starting running weird,it just happened overnight. I thought i had fouled a plug but checked and they were all good. I found out which cylinder wasnt getting power and checked the fuel injector because the spark plug was good. The fuel injector didnt light up with the noid set i got so i umed it was the fuel injecotr but they are only about 2 years old so i had a mechanic check it out and he stuck circuit tester into the pigtail on the fuel injector and it lit up which he said meant it was getting power but the ground was bad. He said that that meant that the ecu was going bad and thats where the problem was. So i want to find out what is wrong with my car before i spend the money on an ecu if its not 100% for sure the problem. Someone please let me know what it sounds like the problem is cause it sucks driving a v-8 with 7 cylinders firing! sorry about the long question but its driving me crazy and i just want it fixed already!! thank you