There are many possibilities for this to happen. Here are just a few: 1 - Clogged radiator 2 - A possible leak in the hose 3 - Low on coolant 4- Cooling fan may not be working 5- Defective radiator...
I had that issue with my 02 cougar xr. It started and drove fine but at stop signs or stop lights it would die. There are a couple of ways to fix this. I replaced my IAC valve or idle air control...