I have the same issue ,especially when going up a freeway ramp in 2nd gear,
It seems to ONLY do that when OIL level is about half way or more on the MY2000 Boxster S .
Have been back to the dealer and they say it didn't happen to them when on a test.
It only seems to happen when I have taken it for a short drive 10 kms or so and then accelerate hard up a ramp.
Could it nbe my Oil level guage is faulty ? Although the dip stick indicator also shows about the same amount on testing it.