2000 Mercury Sable

Hello, I have a 2000 Ford Mercury Sable, 6 cylinder. My car lately has been giving me problems. My caution light continues to blink and suddenly my car will ONLY stay in first gear, if I have my AC on my AC will suddenly turn from cold to HOT. When I turn the car off and sit there for awhile and then turn it back on it will then turn off and let me drive without a problem. I have took my car to a mechanic and they have placed it on a computer a couple of codes popped up. Coolant sensor, fuel pump sensor, pedal sensor. I have changed everything out and my car continues to do the same thing. It's getting frustrated! I don't know what else to do. If anyone else has had this problem OR if anyone knows what this might be feel free to comment or email me in regards to this. Thank you! P.S I thought maybe it was the transmission, but I had that also put on the computer and was told my transmission is fine.