I just had a year of issues with my 3.0 which, after fixing all the routine maintenance issues and the one major problem, it decided to act up on idle, especially at start up. You hit all the big causes but I figure what is left is PCV Valve, especially the vacuum line that goes to it. The elbow on the older models has a tendency to collapse. It does sound like a vacuum leak somewhere, could be the plastic intake plenum. I have also heard that low A/C pressure can effect idle.

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2005 Ranger Edge Extended Cab 3.0 "U" VIN 5 Speed
221,000 miles before the Fatal Valve Seat Regression
2018 New Heads, Valves, Fuel Injectors, Ignition Parts, Alternator, Battery.
Now have a NEW TRUCK for the cost of 2 new car payments.