Can not shift into any gear without heating it up all the way or it stalls. Also when slowing down if the car slightly cools off it will stall. heat her up all the way and keep her heated up and it

I bought a 2000 Chrystler Grand Voyger se minivan. I have to heat up the car until the fan comes on to successfully shift it into reverse drive etc. I can not just jump in the car and put it into any gear with out it totally shutting off. If you don't heat up the car all the way then it will stall when you slow down and shut off. If it happens to cool off at all say on the highway etc. then when you get off at your exit and slow down it shuts off. If the car is heated up and stays heated up it rides smooth. What could this be? When we got diagnostics for it it said it was the mat sensor. We got a new one installed. They plugged it back up to diagnostic and it said the same thing it needed a new mat sensor. No check engine lights have come on or any lights and I have drove it for 300 miles or so.