Why does my car run perfect for about 20 miles then I shut it off go into a store and when I start the car back up it starts acting like it floods itself out and will not start back unless it sets there an hour any suggestions

Suspect coolant temperature sensor sending wrong (low) readings to the computer which will flood engine after it has warmed up due to too much fuel delivery... Try holding accelerator to floor while cranking.. Is the check engine light on?

Also check the Idle Air Control Sensor. This could cause a similar condition. Also, if you have disconnected the battery recently then the Idle Control function on the ECM must be reset by a shop using a T1 diagnostic tool because the Idle control setting is lost when power is disconnected from the system. The car must have and "Idle Learn" function done with the diagnostic tool.