It's a good bet that it's the idle air control valve (IAC). My 89 Mustang 5.0 and my dads 93 Bronco stayed clogged continuously with higher mileage. Not only does it cause a high idle but it also causes it to idle up an down and go dead at stoplights. The reason that it gets clogged is because the crankcase gasses are re-routed through the air intake so that they can be re-burned. With the higher mileage, these gasses start collecting carbon and oil deposits out of the crankcase and depositing them in the intake. This clogs the upper air intake and IAC since it is attached to the intake. IF I'm not mistaken, the IAC is attached to a spacer between the throttle body and upper air intake. It looks like a large dentist drill and has a wiring harness hooked to it at a 90 degree angle. Unbolt this and unplug the wiring harness. Spray it out good with carb and intake cleaner and re-attach it. Be careful not to damage the gasket as this will cause a vacume leak and then you will surely have an idle problem. I'm not a mechanic but this has happen to me several times. Also check the EGR valve. Good Luck!

I'm new to the message board here, so please be patient. If you've got a 5.4 with a rough idle problem, have someone check out the idle air control valve. Mine had an intermittent rough idle problem for several months, mostly when I pulled up to a red light. Then, it began to have starting problems, first cold, then both cold and hot. It finally refused to start for me last week. I had it taken to the dealer who replaced the idle air control valve under warranty. I've had it back for four day now and it seems to run fine now. Hope this helps some of the others who've been experiencing the same problem(s).