My 2001 Ford Taurus SEL 3.0 V6 DOC has developed a problem while idling. The car will vibrate / shake as the revs drop a little, but then the car idles correctly for a few more seconds and then does it again. It used to only do it while the AC was on but now its doing it all the time. Then the engine check light came on and it stopped doing it....however a few days later its back and the car also shudders a little while driving signifying that it is happening at speed and not just when idling. It feels as if the flow of fuel is momentarily blocked. Anyone any ideas what this could be. Ford have just recently replaced the fuel filter, and coolent fluids but I dont think its related. They also just replaced the starter relay. Persoanly I feel like its electrical but I really dont have a clue...any help is appreciated.