Wont Hit 50mph Easily! Help!

Hey guys. What do you think would cause my car (94 Grand Prix 3.1L) to not go over 50 easily? Once I get to about 50 mph the rpms drop. Example. I’m going up the on ramp, I push on the gas, the rpms go to about 3k and shoot back down to idle while the pedal is still pushed, and to make the car go faster I have to slam the pedal down to the floor then there is a 2-3 sec pause, then the car takes off. It can’t be fuel cause the motor wont stall it will just be slow (at high speed), it can’t be spark because the motor will run fine all the time expect on highways, and compression is fine. And check engine light isn’t on. I take care of it, don’t beat on it and have changed fuel filter, air filter, plugs, wires, tranny fluid, ive used a bottle of fuel injector cleaner I just have no idea. Any help would be great.