2000 7.3 running rough at 70mph then loses power

[capt.dru]

My truck starts fine and runs ok, there is a little power loss at the low end. but when i run at 70mph it starts to make a knocking noise and loses power. i replaced inj. #8, and fuel filter. it did get a little better as i can drive at 70 for a bit longer, but still having the problem. any ideas? need help. thanks.