Ok.. im not sure how exactly you're driving your car, but here's what im suspecting:

Under medium/heavy load.. you are driving under partial throttle.(lets say half throttle.) Your car builds some boost, then you proceed to depress the accelarator (to lets say 1/4 throttle)..

What happened in that scenerio is that you're building boost at half throttle and then you close the throttle but not all the way. This in turn gives the motor less fuel, and you are still under boost, causing your car to jerk..

Hopefully i understood your question correctly, and this helps!

later!