2001 Mercury Marquis The engine surges, then cuts off;

Engine problem
2001 Mercury Marquis Engine Size unknown Wheel Drive Type unknown Automatic 6500 miles

When you turn the engine on, you have to run engine about 15 minutes, then the engine surges and cuts off. It use to idle high and then low while running the engine in park, but, now after 15 minutes of running the engine in Park, it cuts off. You literally have to run the engine for about 30 minutes with your foot on gas padel for about 8 minutes in order to keep the engine idling at normal speed while in park. This has been going on ever since my father-in-law gave my husband and I this car a year ago. He had told us that this was happening shortly after he purchased it brand new in 2001. The car has been in and out of dealership so many times it is ridiculous. We are always being told that the dealership cannot find anything wrong with it. We sent it to our mechanics and they have said the same thing. They would clean the oxygenation sensors, run the diagnostics and tell us that they could not find anything wrong with it. Yet the car surges badly and cuts off if the engine in not moving.