i have a 91 firebird 3.1L V6
when im traveling down the freeway i noticed my car shakes at 70-75 mph but if i go to 80mph it stops or if idrive 0-65 it doesnt shake only at 70-75 mph .. now i replaced the shocks the struts
the rotors the bearings in the rotors the tierods the balljoints and the tires i also got my wheels balanced and an alignment
so what could it be??? i thought maybe the bushings??

also if im at a complete stop and im gonna drive off when i step on the pedal and give my car a lot of gas the car jumps like the gear is slipping or something..