Stumped

1. 1991 Chevy Lumina
2. 10000
3. Automatic transmission
4. 2 wheel drive
5. Strange sounds - none
6. Fluid leaks - none

I have run out of ideas. The car will run about 15-20 miles then will stall out. If you let the car set for about 5 minutes (sometimes longer) it will start and run. This has gotten worse lately. I have changed the coil pack, it is new. The module. New computer. New ignition switch. Any suggestions would be greatly appreciated. This one has me stumped. Thank you for your time.