I drive a Renault Kangoo 1.2 petrol 2004 model?

the car seems to be over revving even with no foot on the accelerator and broke down at a busy junction RAC called and got the car going again. Car booked into dealer unable to find anything. Same thing happened again when I stopped at a junction. This time I turned the engine off and got it going again after e few minutes. The car has also has spells of being sluggish. Any ideas what this might be ?