C180 k engine slows down and pick up speed again in a kick mode.

[Yan]

I got a mercedes benz c180 kompressor 2002, few times while am driving the car slows down with my foot on the gas pedal. Then it will slowly speed back up again (it is worse in gear 1,2 and 3 where it goes to kick mode). Took it to the garage for them to check spark plugs but all ok. Did anyone know what is my problem?