Problem with accelerating on highway.

I have a 2003 Chevy Malibu. Here it goes.

Car is having trouble on the highway, and strange enough, ONLY on the highway. Doing city driving the car drives just fantastic. Once I get on the highway the car feels sluggish to accelerate to get up to 70 mph. If I need to pass someone the engine seems to hesitate when I press the gas pedal and for some reason the RMPs will sometimes seem to refuse to go above 2000. Unless I really lay on the gas pedal, the RMP will seemingly freeze at exactly 2000 while I >slowly< accelerate 1 mph at a time. I have: a brand new fuel filter, new air filter, cleaned MAF (that helped initially but now it is going back to the same bad performance), professionally cleaned throttle body of carbon deposits, new muffler and tailpipe along with a very recent oil change, new PCV valve and radiator flush.

Not once have I had a check engine light come on during this whole ordeal (which is frustrating) I've also had a trusted mechanic drive the car with the computer gauge plugged in and as far as he can tell the car is running and reading perfectly fine. Any idea's?