92 Bronco 5.0L gutless and bucking

My 92 Full size has been pretty gutless since I got it a year ago. The last guy put a "rebuilt" 5.0L in, I think from a crown vic or something, all I know is the bronco plugs wouldnt fit, had to get a set of car plugs.
Anyway its always been okay, but not alot of response when giving gas. It feels tourqe'y but very slow.
Ive recently had some issues with major bucking when Im accelerating and even a backfire once in a while. If I give a little gas its not bad, lots though and it craps out (bucks), If I build up to half throttle it runs alright.
I took off the IAC, cleaned it, and it seemed to help a bit, but still same symptoms.

Auto Zone and Advance will scan the engine for computer codes free. I would do this and then tell us the codes.