Re. Steele post #12: I hesitate to comment, not having any experience with the PDK tranny, however, in the interest of offering a 'possible' ave. of trouble shooting...
I had a similar trans problem on a BMW 850 some years ago, and they worked on it for a year with no result. Finally they flew in a 'roving US specialist". Solution, a "star" connector (5 different sensor wires come together in a 5 point plug) to the onboard computer had a loose pin. It made intermittent contact, and not always with the same pin, hence no consistent fault code. Sensors were all good, wires were all good, computer was good, transmission was good...just got intermittent bad signals from the plug. Replaced the connector for a few dollars and fixed it.

Don't know if this is even plausible, but...it's a thought.

Good luck.