I bet its going to be the oxygen sensor...
Idling high, then low, then high? These are noted to go bad and they match the symptoms you are having and this is why:
The oxygen sensor is located in the exhaust pipe right in front of the cat, and can detect rich and lean mixtures. The Oxygen sensor uses a chemical reaction that has voltage as a byproduct. The sensor has a tube made of ceramic zirconia. The zirconia generates voltage from approximately 1V in richer conditions to 0V in leaner conditions. The engine's computer looks at the voltage and determines if the mixture is rich or lean, and adjusts the amount of fuel entering the engine through the fuel injection system.
When the oxygen sensor is worn or broken, the computer does not know what your fuel/air ratio is, so it ends up guessing. Your car runs like crap and uses more fuel than it needs to. I was off roading and actually ripped mine completely out on a tree. The Jeep would idle real high, then real low , real high, real low. I though something was really messed up. I looked under and saw it hanging.
This is also up on my webpage under performance mods if you click the link.
Thanks for the compliment ArmyYJ!
04' Dodge Ram
"Jeep mechanic wanted: must be able to put square peg in round hole"