Sorry to hear that you have an issue with your Nitro. First off I don't like the response from the Dealer you contacted. I would recommend you find another. I don't think the engine needs to come apart. Have you run a compression check? Run some gasoline anti freeze to make sure you don't have some water or bad fuel. Are you running the correct grade of fuel?
I'm surprised to don't show an error code, and hopefully that is a good sign! I believe it must be something simple. You may be surprised but how is the battery? New would be great. They are usually only good for 3 years and a bad one can cause all kinds of issues.
Here is a LINK
Dodge Nitro :Battery issues?
Also if you have not already check out 4.0L ENGINE- Service Information - Nitro
Maybe something will jump out at you there.
Good Luck and let us know what you find.
Nitro Year: 2007 (1 of 91,815 sold in 07)
Nitro Model: R/T 4X4 Stone White
CAT-BACK Exhaust, CAI, Projector Head Lamps
Fully-Equipped w/all factory options