general problems of the 2009 Mercury Grand Marquis

I was driving at 45 mph and applied the brake to stop. The engine revved to 2,000 rpm. I was able to stop with very hard braking using both feet. The engine continue revving at 2,000 rpm even after the car had stopped. I placed the transmission in neutral and the engine revved even faster. The engine did not slow down until until I turned off the ignition. After leaving the engine off for around two minutes, I restarted the engine and it resumed running at around 500 rpm. My 23 year old son was seated next to me and verrified that nothing was touching the accelerator while the engine was revving at 2,000 rpm and that both of my feet were on the brake. The engine has revved when I applied the brake at least seven other times; however, the revving stopped after one or two seconds. I have taken the car to a Ford dealer three times for this problem; however, they have not been able to find the cause of the problem.