car stalls when stopping on a red light or waiting to turn on a stop sign

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after driving on a highway and exited, when i stopped on a red light, the idle speed dropped and the engine stalled. This happened twice. Also when i was driving on a highway and hit rubbernecking traffic, the engine stalled. My nephew who used to work at BMW dealership checked the disa-valve and the fuel fuse which are good. He suggest to replace the idler control valve but it is very expensive. May i know if we are on the right track, or is there any other underlying problem. Please let me know and thank you so much.