My son drives a 2003 Ford Focus with 183,000 miles on it. At random times when he starts the car, the engine runs very rough at idle speed but when he drives away, it runs fine until he slows for a stop sign/turn, etc. and the engine is at idle speed again. When this happens, the engine runs so roughly, that he loses vacuum to the brakes and his brakes stiffen dangerously. The car may not do this at all for a couple of weeks of driving, and when it doesn't do it, it runs beautifully. Also, it only runs like this if it does it from start-up; it never idles roughly in the middle of a drive in which the car started normally. I have changed the spark plugs and wires, PCV valve, fuel filter, and air filter, but it still goes through this about once a week. I bought the car used just a few months ago and it had just had the valve cover gasket changed because the valve cover was leaking. I don't think this plays a role in the above-mentioned problem but thought I would add this info if it helps.