Odd Transmission Problem

[Sir_Ben]

Having had a 911 (964) Tiptronic a few years ago, I recently bought a 2004 Boxster 2.7 with a tiptronic box, with only 13k miles on the clock. After a month or so I started to get a fault that I can't find described in any of the online forums.
When I start up from cold, I reverse out of my driveway - no problems, I select Reverse, the red "R" light displays and the gear engages. When I move the auto shift into Drive I get the "D" light flashing but with no indication of the gear I am in. The car drives and shifts automatically as it should, but the "D" light just keeps flashing. Most times It will eventually display normally, but not always.Some experimentation shows me that:
1. Shifting the lever left into the Manual position allows me to change gear with the buttons on the steering wheel (but the "D" light just keeps flashing.
2. Sometimes, if I build up some speed until I'm in the higher gears, if I shift into "N" then back into "D" I can get the flashing light to disappear and the display to work normally.
3. When it goes to normal display it usually stays that way untiil I stop. Then the whole pantomime starts over again.
4. There is a more worrying feature though, and that is when I am accelerating hard, it will sometimes feel like it is losing drive, before it kicks back in again. I feel that it is more like an electronic inhibitor kicking in than a mechanical transmission/engine problem, but I may be wrong.

I'd be really grateful for any suggestions before I make the 200+ mile round trip to my nearest Porsche dealership.