While driving car starts shaking, jerking and vibrating

While driving either on city or freeway the car started shaking, jerking, and vibrating really badly. When applied pressure to the accelerator pedal, the vehicle would not gain speed, instead it would slow down and it felt like gas was not going through. It first started three months ago took it to for an oil change and the recommendation I received from service agent was to get fuel system cleaned and change the transmission fluids which cost me around $298.00. The same day it did the same thing while driving on the freeway, so I let it be for a while but it continues with the same problem and have contacted service agent but have no solution for me.