Car lacks power on take off

I have a 95 ford mustang with the 3.8.
you have to put it to the floor to get it go but once you get it up to about 40 mph it does fine to you hit a hill then you have to floor it again to keep speed. I have changed the coil pack, spark pulgs, computer, mass airfloe sensor, and fuel pump. I dont now what else to do. The scan tool says p1131 could the oxygen sensor cause these problems