Hi
Have the speedo problem looked at / fixed & you will most likely find the cutting out problem will be fixed at the same time.
If the car doesn't know its actually driving along the road then the ECU cant raise the idle speed to stop the engine cutting out as you slow down.
Either the speed sensor on the gearbox or if its a later multiplexed & ABS equipped 206 the speedo runs off the ABS sensor on the left front wheel.
What year is this 206, what engine size & fuel type & also does it have ABS brakes fitted?
Lee
June 2007