Cayenne going into limp home mode - recurring problem

[Jockymcn]

Anyone experienced a similar problem, successful diagnosis or satisfaction with Porsche?
66 plate Cayenne diesel goes into limp home mode every 3-6 months. Only warning is coil light on dash starts to flash beforehand.
Car has been to the dealer (recovery truck) numerous times. Possible reasons given include low battery, defective exhaust valve and ignition problem. However, fault seems to mysteriously disappear every time when it reaches dealer and car gets returned with me being told that they are unable to identify or diagnose the problem.
Am wondering if something to do with emissions / DPF which they don’t want to acknowledge.

[Twinfan]

If it's an S model diesel it'll be the particulate filter bunging up. Do you perhaps do lots of short journeys? If so, you need to take it out regularly for a long journey at speed to get the car really warm and burn all the particles off.