My problem happens when engine is warmed up, idling at stop, but not in park, only in gear and neutral. Started a month ago. When in park, there is a little bit of a rough idle, shaking, but smooths out fine when put in gear. The rpm's will suddenly drop to 500 or so as if it is stalling but quickly recovers after a second. Does it about every 5mins when at stop/idle, not when driving/accelerating, although seems to have liitle less power when driving. No check engine light or codes. Spark plugs seem fine, changed 2 years ago. Coil pack replaced, Motorcraft, 2 years ago. MAF, IAC, Throttle Body and MAP were cleaned, oil changed, but made no difference. Smoked tested for vacuum leak in engine, but didnt find anything. Thought i heard brake booster hissing, but brake pedal seems fine. Not sure what else might be the cause. Normal idle rpm for this Ranger, with 120k miles, has been around 850-870, a but high i think, a little shake/misfire in park but not horrible, and no codes.

Checked fuel pressure when engine running, got 61psi. Brand new Motorcraft fuel filter put on, but made no change. Any thoughts?