Is the car an automatic transmission? Does it only do it in gear? Possibly the idle air circuit (IAC) is gummed up and not responding quick enough, so the car is stalling when you stop. By the time you take the car out of gear and engage the starter, the IAC has managed to open up the the start position. If it does it anytime you step on the brake you must have some weird electrical cross-talk that could be the devil to find. I would check the brake lights for a good ground (less then 1 ohm), then you need a good Peugeot mechanic.
Blacksmith