After driving for a period of time and coming to a stop ,say at the bank drive up window the car goes into a rolling idle. And also at cold start up. Up and down for about 5 or 10 minutes . Thought this to be normal seeing how it was a cold start but when I drove it for a period of time and it still did it I was concerned. And one more thing, it doesnt do it all the time. My salesman who is an avid mustang scholar said it is the motor staying efficient. As much as I like my salesman I cant seem to agree with his opinion. And he has been my salesman for 3 car buys now. Thanks for any help.
Rolling idle in 2005 GT 
