1991 F150 check engine light on

My son has my dads 1991 f150, 4.9 liter, automatic, 2 wheel drive farm truck as his dream machine. We have replaced the EGR Valv,e Egr Sensor, EGR Solinoid, O2 Sensor, Throttle position sensor, temperature sensor, and air flow sensor the last 2 are up front within a couple of inches of each other also the MAP Sensor got replaced, I noticed it ran better after it was changed. I have also ran a rod down the egr tube to make sure it was not cloged up. We took it to Ford and they said three of the new items on the truck were bad so off we go to Oreillys and Autozone and they gave us new ones, we still have a check engine light. After we replaced the new items the light was off my son drove maybe 25-30 miles and it came back on. Ford and our mechanic has said the injectors need to be cleaned which will happen this Thursday. Our machanic has suggested it could be the computer or wiring. The truck seems to run pretty good at least alot better than it did. Anybody have another suggestion.

Take the truck to autozone or to your mechanic and have them pull the codes and see what is making the check engine light be on.