Ford Repair: Shifting Problem?, rear brakes, ford mustang

Question
I have a 2000 Ford Mustang with a 3.8 V6, and a Auto Transmission. It currently has 80k mileage. My problem is that when I turn th A/C on and the car has been driven for awhile it seems like the car is trying to downshift, but cant or wont. Also when I come to a complete stop the car almost dies, and when I get ready to go again it seems like the rear brakes are sticking because after i give it some gas (more than usual) the rear end makes a poping sound, and the car goes as normal. Also the engine only does this while I am traveling at 40 MPH. any help you can give is appreciated.

Answer
Hello and thanks for the question...I'll try to help.
I would suggest a few things:
1. Have the computer system checked for fault codes.
2. What's the overall tuneup condition of the engine? Does it need to be atended to?
3. Is the timing belt in good condition or does it need to be changed?
Erik