Loss of power between 30-50mph

[kdailey4315]

Hello all,
I need some help. When I'm driving and I reach about 30mph my truck seems to loose a bunch of power and this last till about 50mph. It doesn't cut out or die it just looses a lot of. I changed the cat and it got much better but it is still pretty bad. I had the tranny checked out and it is fine. I changed the fuel filter and gave it a complete tune up wires, plugs, cap, rotor, air filter and still have the problem. It's a gm crate 5.7l tbi engine with about 30k on it. Any ideas what it could be. I'm leaning toward the o2 sensor but I'm not sure. Since it launches from a dead stop fine and idles fine I don't think it could be fuel pump but I could be wrong. Thanks