What would cause a car to stall out after running for a short while?

I had the same problem with a Chevy cavalier. Diagnostics did nothing for me an at a cost of 500 + dollars I chose to go with the obvious. After driving a 15 to 30 minutes car would shut down an not crank and run for about 1 hour. I changed coil packs an ignition module. Runs fine now.These parts new total about 200 dollars, go to your lcal salvage yard, I picked up these parts for 75 dollars, an not that hard to put on. U.