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..

 

sounds like the trans mount is bad

 

Yeah I agree on that part....if the car THUNKS from a standing stop thats definitely a trans mount. I had a blown MAJOR trans mount when I first got my car...when I hit the gas, the entire transmission torqued sideways. Dented my floorpans all up.

I imagine that a trans mount MIGHT cause some vibration, but then I dont know. How are your U-joints on the driveshaft?

And one important question...do you feel it through the steering wheel, or through the car? If it's not through the wheel then most like it's not a front-end problem.

 

My guess has to be with the U-Joints. Climb under the car and see if you can shake the driveshaft with your hands, if you hear clunking, then it's definitly the joints that are shot.

 

also check to see if there are any balance weights missing from teh driveshaft while you're down there.

 

Check the simple stuff first like see if the tires are all balanced. Also a bent wheel may cause that too but they should be able to tell you if ones bent on a balancer.