My erratic idle on a 300TE ended up being a combination of an improperly installed injector boot, cracked and leaking idle air control hoses, old injectors and leaking seats, a loose HVAC vacuum hose, a bad EHA and a bad O2 sensor. You can spray around the intake area with MAF cleaner to see if idle changes. If you spray an area and the idle increases, you have found a vacuum leak. If you have no vacuum leaks, check to see is the EHA is seeping and/or you have a fuel smell coming from under the hood when you shut the engine off. If the O2 sensors haven't been replaced in a hundred thousand miles or so, that may well be the culprit. Those are fairly cheap, if you buy the Ford models and splice the wiring harness.