c230_kS

i just got my car for like two weeks, and it happen twice that when i start my car, the engine was reving fast and slow which made a really werid sound.... and i can feel the car was shaking slightly left and right. can anyone tell me what is wrong with it?Also, when i led the car glide at 20mi without braking and push on the gas pedal again sometimes...the car jerks twice and then move on

i did stop the car by downshifting the gears hard which it says in the manual that do not attempt to do that. does it lead to the problems above?

i really love my car, and it sucks for this to happen i really hope it can be fixed........