4ZE1 Problem... HELP!

I have a RS '89 trooper which I bought seven years ago and it was working well till I had to change the head which was last year. Everything was OK again till I decided to add an aux. fan. When i got it out of the shop i noticed that i was loosing power when i would step on the accelerator. Brought it to another shop and changed the fuel pump, filter, sensor, distributor, high tension wires, etc. I still have the same problem ! Need advice on what the problem is, or what diesel engine I can replace it with with the least modifications.