1989 Cadillac Deville hesitation, loss of power at 55 and 6

This car runs great, I have lots of power to roam around the city, if I punch it at a green light, I have no problem getting ahead of the game. Its not an issue untill I get on the highway, I have no problem getting to 60, but if I want to maintain this speed, it spats and sputters, forget trying to use the passing gear it will bog to almost nothing. The car wont die, I just have to go under 45 to get the gass pedal back. I have replaced the cat. And the muffler, I know it dosn"t have a problem breathing. Could it be the fuel pump, where on the car is it?
I recently took it for an emissions test, and its scores are as good as most of the new cars today. Can you help me?