is this a transmission going? or Catyalac converter problem or TSP or ???

have a 2001 grand am 3.4L V6 - when ever i go 80 KPH - the car will not go any faster when the car RPM's will go up but it will not take off. this doesn't happen all the time.

i can floor the gas pedal and nothing. I have changed the spark plugs, and wires, checked the mass air flow sensor - and replaced part of the EVAP system (solenoid)