Sorry to hear about the problem.
Do not drive your car anymore before the dealer/Porsche sorted out the problem.
Again: DO NOT drive this car again.
I had a similar problem with the DKG gearbox on my wife's M3 Cab, there was a recall later on which fixed it (a software glitch).
Since the Cayenne is already on the market for some time, I suspect a faulty ECU unit.
I had a similar problem, engine shut off completely at 140(!) kph on the Autobahn and it was really difficult to bring the car to a safe standstill afterwards, I was very lucky. The culprit: A cold solder joint (don't ask... ).
Very dangerous situation.
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RC (Germany) - Rennteam Editor Porsche Panamera Turbo S, BMW X5M, Mercedes C63 AMG Coupe PP/DP, Mini Cooper S Countryman All4