I just bought a 2005 2500 about a month ago from a local lot and have run into an odd situation. I have searched on here and found a couple of others that describe a similar situation, but no diagnosis that I can find.

The truck has 214K miles, but other than this issue drives and runs like it has 40K. When in Park or Neutral and you tap the gas, the rpms rise and dont come back down. I have to shut the truck off to get it to return to idle. Another symptom which I dont know if it is related is that when cruising, either on cruise control or using the pedal, when the engine becomes unloaded (let off the gas or crest a hill), it stumbles and studders a bit. The truck had no tuner on it when I bought it, and I borrowed a buddy's Smarty to play with and see if it helped and it does not. The truck appeared to be stock when I got it, and I have added a whistler intake, and had my cat and muffler cut out. I dont know any service history of the vehicle other that a couple of entries on the carfax, but those were just recalls and maintenance items. I took a video of it and put it on youtube to shoe exactly what it does. Any help would be great. Oh and it is an Auto trans 4x4 Quad cab.
Chris

 

I dont know if this helps at all, but my MPG has been substantially lower than expected. Im getting 12-13 on paper while driving conservatively, and the lie box says about 19.

 

I had a similar issue, but not quite to the extent of yours (142K miles on the truck). Ended up being injectors. Replaced all 6 and now it runs like a new truck.