after accelerating to 120+ slowing down and then accelerating again my engine like starts to shake. you can feel it on the accelerater. why does it do that?

 

Leaving it in top gear? Probably because the spark plugs get slightly fuel fouled after a hard run like that going to a long decel period, when you try to accelerate again, the plugs have to be re-heated to thier self-cleaning temperature to have a clean face for a powerful spark again. I don't know specifically for your engine, but that is my initial educated guess.

 

I've did it on mine and have had no problem out of it. The car rides smooth as a baby's ass, almost like it's floating.