My car started doing it this morning, only as it got closer to operating temperature. Noticed when taking off from a standing start it was ridiculously hard to get moving. Tried tiptronic mode, no go, wouldn't respond to my changes at all. Eventually deduced it was stuck in third. Turned the car off, started it again and the A/T warning light on the dash flashed but it drove okay. Started it again later in the day, again drove fine for about 15 mins, pulled up to a set of lights - stuck in third again. Turn the car off and then on again, A/T warning light on the dash flashing.
It's really sudden, there was nothing leading up to this happening.
Reading this thread, I'm getting worried as not everyone's sorted their problems out...