Okay, my car is a 1997 Mustang GT, Manual, with about 60,000 miles. I was noticing tonight while sitting in line waiting to get food that when just sitting my car idles around 600 or 700 RPM's which I'm guessing is normal? Well I was noticing every once in a while that it would drop to around 500 real quick and make a shuttering sound which sounded like it was coming from underneath the dash but I'm not for certain. It only only do it for a second then return to normal. I also noticed sometimes while doing it, my lights would dim for a second. Anyone have any ideas what this could be?