Need help...idle problem

Ok, I am driving my car for about 10 miles on a highway...get off onto a road with lights...come to a light and put the car in neutral. All of a sudden the RPMs go from approx. 700 to 1500(the RPM my car is at when I start it in the AM). The outside temp today was maybe 35 degrees F. I put it back in gear and they got back to 700 or so. What the hell is causing this?? BTW, my idle and timing was set to Infiniti factory specs about 15,000 miles ago. All I have done to the car is a JWT POP charger. This doesn't happen all the time though...any ideas?

Go here: http://www.se-r.net/engine/troubleshooting_guide/index.html get comfortable and start reading.

Thanks, but I have read that and I just brought the car to my mechanic this weekend. I mentioned those things to him, but he could find no problems.