My car keeps turning off when I stop it slow down?

I have a Ford Fiesta 2003 - 1.4 engine -petrol -- So from the past 2 weeks, my car has been constantly turning off when I slow down or stop. However I have no problems driving and accelerating, but the moment I have to slow down at the traffic light or give way, it turns off and I see a yellow engine management sign on the left dash board which goes away with starting the car again. I've noticed that if I give my car race for at least 10-15 min before driving off, it doesn't stall anymore but it starts revving without me pressing gas throughout the journey. I've taken my car to thr mechanics twice for this reason and explained everything, the first visit they changed my battery, and second Visit they said the engine oil was too much so they had to drain it and add a different one. They've charged me both times. But I'm still having the exact sam problem. Can anyone tell me what might be causing this and should I take it back to them or somewhere else

If you are listening to lube boys where you get your oil changed I'd not go by what they tell you.
You need to first clean the throttle body. Then if it keeps stalling you need a mechanic with a $2000 scan tool to hook it up to your car and read the engine data to make sure all of the sensors are working with in spec. Then they need to check that the idle air control motor is working

Take it somewhere else.
My first check would be the Idle Air Control valve.