If I understand your problem correctly, it seems to be a faulty "idle speed" valve. If you take your fiesta to a ford garage, they'll wire it up to a computer, and this should identify this as the issue. To replace one costs about �50ish, but it regulates the speed that the engine ticks over etc. When I had a problem with mine, the engine cut out at traffic lights and when slowing down on the motorway etc. However, theres a possibility yours might just need cleaning......if so, bobs your uncle, problem solved!