i have found that on the 4-cylinder intake manifold there are a set of butterflies INSIDE the individual runners. these are controled by a vacuum assisted arm that opens and closes them. this arm itself is made of Plastic, thus it breaks and/or wears out easilly. when this happens it keeps the butterflies closed. which means about a 75% airflow reduction. there are no forums on this so i figured after 3 years of dealing with this problem and finally figuring it out, i would post this for you all. mine actually fell off the kuckle and was stuck closed all this time. i just thought i bought a lemon. ford garage...THREE of them, hooked up to the computer and told me "nothing wrong with it. it's the Nature of the beast" what a load.[:@]
oh mine's an '01, so i hope this helps you all out on surging, poor/rough idle, bad gas mileage, etc.

**if anyone knows of a replacement manifold other than stock for a fuel injected system let me know Please**