boxster's transmission help

[bonvita1973]

hi everyone, i own a well kept 1997 boxster with only 26000 miles, mine is a tiptronic. while i was driving on the highway, at about 70mph my car out of a sudden, just had a quick jerk as i accelerated to pass another car. Out of a sudden my 4th gear was flashing and my speed had drop to 65, I drove for a couple miles, got off the road, turn it off and did a restart and the car was fine for about 6 month and the car just did thesame thing again. took it to our local porsche dealer, and they couldn't find anything wrong with the car.The manual says the car goes into 4th gear as a safety measure, to protect your transmission from any damage.Well,my dealers hasn't being of any help, so any good ideas or have any of you experience this? thanks kindly for the help