MY 2010 RANGE ROVER HSE GOES DEAD WITH OUT WARNING MOST OF THE TIME AT ABOUT 20

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THE CAR GOES DEAD AT ABOUT 20 MPH AND WILL START RIGHT BACK UP AND OF COURSE YOU LOSE ALL POWER STEERING AND BRAKES. IT STARTED AFTER ABOUT 70,000 MILES IT NOW HAS 143,000. I HAVE HAD THE CAR AT THE DEALERSHIP AND A PRIVATE GARAGE AND SPENT SEVERAL THOUSANDS DOLLARS AND NO ONE HAS BEEN ABLE TO FIX IT OR EVEN A GOOD GUESS AS TO THE PROBLEM.