Why do my RPM's fluctuate when I put cart in neutral while coming to a stop?

What is your question?

I have a 2007 Ford Mustang GT, that when I put my car into neutral when coming to a stop, my RPM's fluctuate which makes my engine rev and when I come to a stop it usually settles down and the RPM sits where it's supposed to at 500. It doesn't do this every single time I put my car in neutral. When it first started doing this, it only did it when I had the AC running, but now does it even when the AC is not on. I thought maybe it was something to do with the AC so brought car to mechanic and they found a hole in one of my AC hoses so they changed all of that and also had them change all of the spark plugs and just picked up my car two days ago and today it started doing the RPM thing all over again. I do have a tuner programed on the car could this be causing the problem?

What makes this problem better or worse?

not really sure .... it just does it at different times

How long has your 2007 Ford Mustang had this problem?

a few months