Why is my 1996 std Mustang pulling in every gear like if the motor was cold?

I changed the plugs, wires, ignition coil, air filter, fuel filter but it still pulls as i'm in all gears until it gets ran about an hour. I recently noticed that it feels alot heavier than other vehicles that I have driven that are also manual transmission, it doesn't have that imidiate take off in 1st hear anymore. The motor is still very strong though with no knocking. PLEASE HELP ME!