My car has been doing something weird, and I can't figure out what's wrong. I have a 2007 Mustang GT with a 91 octane performance tune, hot rod cams, 3.73 gears with limited slip and a cat back exhaust. Every time I come to a stop, I put my car in neutral like usual, but my RPMs will rise up to 1100 at idle, and then after about 5 seconds, it will drop to the regular 850 RPM at idle. Even if I put it into neutral at 1000 rpm it will still rise. Then, if I let my car just coast down a hill...

Weird idle hanging problems