idle issues

Hey guys got a little problem. I have a 99 mustang GT. Ok whenever i start my car start the car the idling is fine its a between 500 and 700. When I put the car in reverse or drive the car idling goes way down like around 200 to 400 rpm and then it builds itself back up to normal after a sec, but if i drive some it goes down until i press the gas some. Also when I drive it goes fine no problem but when I start to slow do before i make a complete stop the car has idled down to 500 already and when i make a complete stop it goes lower like it wants to cut off. I do have a exhaust leak but thats not been an issue it all started after i fixed my speedometer with a programmer. Could someone please help me out here. email me @ crumpler03@yahoo.com or just reply to this please. Really need to fix this issue

Your Idle Air Control (IAC) Valve is a likely culprit. It is on the back of your intake plenum. It is held on by 2 10mm bolts and has a small air intake tube going to it, so should be easy to spot and remove.

Some have success with cleaning it with carb cleaner, but usually ends up being only a temporary solution if it fixes it at all. You can get a replacement from your dealership or get a Duralast part at Autozone for about $80. I got a Duralast one and it has worked well so far for over a year and a half. It also came with a lifetime warranty which is a plus.