throttle positioning sensor

we just put a 89 v8 5.0 ford racing engine in my 94 mustang. And when the gas is barely on just enough at crusing speed at 1500-1600 RPM it misses and stumbles. we already try getting a new sensor.. and nothing... and drilling the holes out of the old one so we could move it more to see if we could stop the stumble. its gotten better but its still there. so any suggestions will be awesome.