Ford Ranger / B-Series :: 1999 - Stalls / Rough Idle And Engine Dies

I have a 1999 B4000 4.0L with 160K miles on it. Bought the truck new. Up until about a year ago it ran perfect but has developed an idle, stalling and occassional jerky ride problem. It occurs at any temperature, engine hot or cold, A/C clutch on or off and in Park, Drive or Neutral. When you start the engine it will not idle unless you keep your foot on the gas. The idle speed will drop to 200-300 rpm and the engine dies. Sometimes you can barely tap the gas pedal and the rpm will shoot up to normal idle speed (about 900 rpm) and stay there a few seconds then begin to drop again. If the engine is cold it will often die a couple times after starting even with the foot on the gas and then about the 2nd or 3rd time run with your foot on the gas.

At stop signs and red lights you have to keep your foot on the gas to keep it running. If you are approaching a stop sign and take your foot off the gas to coast to a stop and brake, the engine rpm will drop and the engine will die. It always starts fine though. Once you are driving it generally runs just fine at low or high speed. You can generally set the cruise control on the expressway and cruise right on. However, sometimes all of a sudden the truck will start bucking and jumping as if it has a terrible misfire. It usually lasts a few seconds and quits. Often if you step on the gas it quits. Even less occasionally the engine will begin to slow as if it is starved for fuel and slow down. You can press the pedal to the floor and it sputters a bit then picks up and runs fine.

I have read about all the threads in this forum and others and tried some of the suggested fixes but it still has the problem. I have tuned it up with new plugs, wires, fuel filter, air filter and PCV valve. Based on some of the forums advice I have replaced the IAC valve, the EGR valve and EGR Pressure Feedback Sensor, and cleaned and inspected the MAF sensor with MAF cleaner. I have checked all my vacuum lines and no leaks there and checked the engine timing which is fine. Fuel rail pressure and engine compression are also fine. Spark plugs show no clues, exhaust is clean and the engine uses virtually no oil. There has been no tear down on the engine like removing the heads, cam, timing gears or any of that and the engine is completely stock.

It shows no codes related to the problem. Two or three months after the problem started I showed a code that indicated an injector problem, traced the problem to a bad injector, replaced the faulty injector and it was fine. That took care of that code but didn't solve the idle problem. In my younger days I was into drag racing and I rebuilt engines, swapped transmissions, etc. and still do my own tune-ups today so I am fairly familiar with auto repair and have most of the tools for testing newer cars but this one stumps me. As a last resort I'm about to take it to the dealer.