I bought a used Mazda 3, mechanically have not had to fix anything since bought. Past few days I've noticed when I start my car, then put it in drive my RPMs will go down to about 500. When I put it back in park it will go back up to the usual 900ish. When I am driving the car and come to a slow down and stop, my RPMs will bounce, then will go down and I will stall out. While I am driving the car is okay, only when I am coming to a stop or have my foot off the gas I have this problem. When my foot is off the gas, my RPMs will go down and bounce, when I apply my breaks the RPMS will go down and she will stall and shut off. I have checked the fluids. I have checked the sensors. I have checked the air intake filter. I have checked the battery and altonator. What could this problem be?