Looking For A Mustang Shop

I'm on the other side of the country so I can't help with recommending a shop, but as for the problem you're having, a friend of mine had a similar one. Start your car with lights, radio, and whatever other electronics off and let it idle, then turn on all your lights and radio and see if your rpms drop quite a bit or if the car starts idling low. If this is the case, it's probably your alternator and you should get it replaced. If you're not sure, you could take your alternator to an auto parts store and have them check it to see if it needs to be replaced and you could check your battery while your at it. If you have a multimeter you could check them yourself. Alternator should be around 13.2v - 14.8v. Battery should be around 12v, someone correct me if I'm wrong.

The alternator is supposed to supply power to charge the battery and if the alternator isn't supplying a sufficient charge, the car could possibly idle low or stall, especially at a stop.

The check engine light and battery light comes on after the car shuts off because the key is in the 'on' position.

Hope this helps

Damn I just closed my shop in Orlando and moved out of state. PM me and I can give you a couple good places to go and a couple to stay away from.