I have a 98 xj with a 4.0/ax15 and if I sit idle in neutral with my foot in the brake she starts to misfire but won't stall. I thought it was a vacuum leak but it does it at all random times and revs on its own then it idles up around 3000 rpm slowly dropping off to normal idle.

 

First thing I would do, is clean the throttle body and IAC. Get all the carbon build up cleaned out of there and see if that helps. Cost is $5 for a can of throttle body cleaner (good to have around anyways), and a half hour of your time.


If that doesn't correct it, then its time to start trouble shooting other possible causes... but a dirty TB and IAC can certainly cause idle problems and is an easy place to start looking.