Fairmont shuts down on hot days

My 2001 Ford Fairmont just stops in traffic on warm to hot days after two to three hours driving. Wait 10 minutes and it starts, but it stops again after 200 metres. Imagine this happening in peak hour traffic. When the air temperature cools down, I can get to my destination without any problem. I can drive throughout the year without this happening, but come summer it starts again. Last week the mechanic replaced the coil pack but the car stopped again this week.