I started having a weird problem with my 2006 Fiesta ST when cold starting the engine, it starts up with no problem but then the revs drop low like the engine is going to die and then surge back up and it does it about two times. But after that engine idles normal and when driving I have no problem with idling. I have already cleaned the throttle body and unplugged battery which solved problem for about 3 days and then problem returned. I also have pretty new spark plugs, battery, fuel filter and air filter. Any idea what is causing the surge when cold staring engine?