2002 Mercury Sable RPM

Last summer I have parked my car to my garage after a long trip. The weather was hot, 90 F. In ten minutes I started the engine, intending to make another trip, but the engine just died: RPM was not supported at "Parking" position of the stick. Tried another time, and another time. Battery is strong, starting was not a problem, but RPM at idling came to "0" soon.
Next morning I started the engine and ran it at high RPM for 5 minutes. Then gradually reduced to normal. It worked OK. Brought it to a good repair shop for inspection, they could not find anything wrong.
Last week I got the same problem after six months of normal operation. RPM was not supported at "Parking" position of the stick. Two lights came up: "Charging system" and "Service engine soon". My car was towed to a very experienced repair shop. In two days they started to inspect the car. Could not find anything wrong.
Any idea of what is going on?