2002 Peugeot 307 1.4 petrol style - Stalling 307 - janeyg

I bought my peugeot from a dealership in December gone and had nothing but trouble but my main concern is that the car stalls when I approach a junction or roundabout. Its not all the time but enough for it to become a pain. had it back to the dealer who tells me nothing is showing on diagnostics, they say it doesnt have a idle speeed control (hm???) and they cant find a fault as when they take it out its fine. IT OBVIOUSLY ISNT! Took it for a 2nd opionion today and they say it may be the stepper motor, can anyone help I have no idea what these parts are and being fobbed off by dealership.