I also posted this in the Cayenne forum.
My 2012 Cayenne S has 13k miles. Last week the "Reduced Engine Power" Warning started flashing on the display. I took it to the dealer immediately.
I bought it in August and it's been back to the dealer four times.
They kept it for two days and told me they couldn't replicate the problem and assumed I had either: a.) put in non-premium gas or b.) got bad tank of premium from the local BP station. My last four cars have required premium gas so I would not make such a mistake.
The next day, I was driving to the airport and the "Reduced Engine Power" warning and chime began to go off and on again every few seconds like a fire alarm. I called the service manager (who is a great guy and gave me his cell number.) and told him I was ready to "give this piece of junk back to Porsche."
He had one of his porters meet me half-way to the airport and swap my Cayenne for a Panamera 4.
Now I just wish I had bought a Panamera. The car is awesome. I drove it through a snow storm and it stuck to the road like no car I've ever driven. It handled snow covered roads as if they were completely dry.

 

Sorry about your troubles and I hope this is fixed soon.

You should insist you are not putting in lower-grade gas. Ask the dealer what specific gas station they use to fill up their loaners and use that gas station if it is practical (and save your receipts for later).

If the "Reduced Engine Power" displays again, it is possible to drive directly to the nearest dealership to diagnose further?

Did the dealer have any other theories as to what else could be wrong?
Isn't there a knock-sensor in the engine to check the gas quality? If so, shouldn't the mechanics read the codes and determine what may have happened.

I am disappointed the mechanics are unable to review past history and codes from the on-board computer to determine what broke.

 

Sorry to hear This. They told me a fault has to happen 3 times before there is an error code...

 

If you are able to pull over, record the fault with your cellphone. Each time it occurs be sure to have the date active on the phone, so that you can prove it occurs.
Once you have that then you don't need to prove that the car is having issues.
Then you can request a resolution to this issue that occurs periodically.

 

Panamera is a great car, no it is an AWESOME car. I drive it more than the 911 now.