I WAS DRIVING MY NEW 2005 PORSCHE CAYENNE TURBO PURCHASED AT THE END OF DECEMBER, 2004 ON THE HIGHWAY AT AROUND 50 MPH WHEN SUDDENLY THE CAR SHIFTED TO A LOWER GEAR AND JERKED FORWARD WITH SUCH INTENSITY, IT WAS AS IF SOMEONE HAD PUT ON THE EMERGENCY BRAKE. THE "EMISSION CONTROL, CHECK ENGINE" LIGHT CAME ON IMMEDIATELY AFTER THIS OCCURRED. THE INCIDENT CAUSED WHIPLASH BECAUSE THE CAR JOLTED SO SUDDENLY. IF THERE HAD BEEN ANYONE DRIVING CLOSELY BEHIND ME, THEY WOULD HAVE CRASHED INTO ME. THE CAR WAS TOWED IN FOR SERVICE TO A PORSCHE SERVICE CENTER AND AFTER SEVERAL ATTEMPTS AT DIAGNOSING THE PROBLEM, THEY HAD TO REPLACE THE DME (THE ENGINE MANAGEMENT COMPUTER). THE SERVICE CENTER STATED THIS WAS AN AUTOMATIC FUNCTION OF THE VEHICLE, THAT IT AUTOMATICALLY GOES INTO 3RD GEAR IF THE ONBOARD COMPUTER SENSES A PROBLEM. IF I HAD BEEN GOING 70 MPH ON A NORMAL HIGHWAY, THE CONSEQUENCES WOULD HAVE BEEN SEVERE.