Ford Focus cutting out when stationery

Hi all, wondering if you can help?? I brought this ford focus 1.8 lx a couple of months ago and have had problems with high and low rangeing ever since (ran like a dream before supposedly). anyway i have had a new speed sensor fitted, new hose to the idle control valve and put on a diagnostics which showed a new throttle sensor was needed, which was then replaced. unfortunately i am still having the same problems, my car is idleing between 200 and 600 rpm up and down, this is causing juddering and the battery light comes on and off. eventually the battery light will go off and the car stalls. It starts first time everytime so i am hopeing that it is some kind of power issue?? and not a new ECU job! any ideas??