My truck is driving my crazy! I've been trying to figure out what is wrong with my truck. It has a rough idle. Pulsates between 500RPM and 1500RPM when started cold. Its goes up and down for about a minute before the RPMs level out. Sometimes it just dies. Sometimes after driving for a while and i throw it into neutral to began to slow down, the RPM's stay up around 2500 for about 5 seconds after i've came to a complete stop. So far within the past year I've been replacing parts here and there, they include:
-ignition coil
-plugs & wires
-starter
-fuel pump
-fule filter
-MAF sensor
-O2 sensor L & R
-Air Filter
-Fuel injectors
-Coolant/air temp sensor
-Throttle position sensor ordered but not installed

Please help me figure this out. I've thought maybe it is a gasket or maybe I did something wrong while installing the fuel injectors, I just don't know and I want to stop wait sting money!

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Ranger 4.0L 4WD XLT 5mt