2012 Ford Fiesta SES

i have a lime green 2012 Ford Fiesta SE (not the stick shift model) that i fell in love with that i bought used from a hyundai dealership and it's been having a problem that whenever i stop at a red light, it wants to go out as if i was stalling. i layed down half of the money at the dealership the day i test drove it (making a deal that they would fix it) and around a week later they called me and i came in. it drove like a dream. a couple weeks later, the problem came back. my apologies if there is already a thread for this for i couldn't find it. has anyone else had this problem and have you gotten it fixed? thank you in advance <3
You are not giving us much to proceed on...

to make an even half assed informed guess regarding what is going on with your fiesta. But I will take a shot! I will assume this car has over 80 thousand miles on it. I will assume it does not have any aftermarket hi amp audio equipment installed and I will assume It has never been in an accident that required replacement or removal of the electrical harnesses and or fuel system.
You do not say if it runs well and this condition just occurs randomly or if it is an ongoing constant either low rpm chugging or drop off of rpms at idle when at a stoplight. you also don't report what the dealer service tech listed as the actions he took to diagnose the condition. so here goes. if the car spent a lot of time being driven with less than 1/4 tank of fuel by its previous owner, the fuel pump my well be saying goodbye. if the dealership checked the line pressure at the rail it should be constant while the engine is at idle. If it fluctuates, there may be a floating obstruction in the( built in) filter on the pump. or the pump is failing. Is there a loud humming sound emanating from the tank area? The pump is a design that does not have fuel return lines to the tank. it takes demand signals from the pcm to pump more or less fuel depending on your throttle position. So, possible fuel pump or plugged supply at the filter. Did the dealership check pump pressure?

The other high probability cause may be a very gummed up throttle body. did your dealer say they looked at or cleaned it? The throttle plate allows intake air to pass through at idle, unless there is a lot of ******* blocking the opening. Again, not having much information on the specific situation this occurs in and the odometer mileage/average mpg/when it was last serviced etc. I am just guessing.