1995 Mitsubishi 3000GT Noticing a loss of power

Hi I own a Mitsubishi 3000gt(non Turbo)(Manual). Lately iv been driving up hills and when I push the gas pedal clear to the floor it seems to just wants to die on me, as soon as I let up on the gas from all the way to the floor, it picks back up speed, even when im on the highway if I accelerate to the floor it really does not want to pick up speed, when I first bought the car it had 158,000 miles and it had no problem picking up speed or have no problem going up a hill until I owned it for about 3 months, some of my buddys says it could be a Catalytic Converter, fuel pump, spark plugs, fuel injectors, I already replaced my fuel pump and my spark plugs and they did not help the problem, but I also noticed when I get gas cause its premium only it seems to bog down when I take off after putting the fuel in, id figure it was just water in my tank so I bought fuel injector cleaner and I did not help, it still does not pick up speed as it used to.