EDIT: aerohead already posted even better information about nose-to-tail drafting!
OK. Then I would be interested to understand the physics behind the "side drafting". When the physics is understood, then the solution to prevent it could be found.
I believe the nose-to-tail drafting advantage is well proven; a good discussion is here:
HowStuffWorks "How NASCAR Drafting Works". While the article covers NASCAR, the drafting advantage to the front vehicle is not limited to NASCAR.
So it seems clear from what is known that you will notice a sudden increase in drag when a car which is drafting you pulls out from behind you and tries to go next to you... especially at 215km/h.
But it is not clear what is happening when the car is coming next to your rear wheel. What might be happening, if the other car is very close, is that it is disrupting the smooth flow of air along the side of your car, creating even more drag than normal. Maybe this is your 2nd noticeable increase in drag (it is not the engine braking hard, but the aero drag increasing which means more engine power is needed to keep the current speed)