[911jon]

The 911 body shape is a complex one, subject both to lift (like almost all cars) and sidewind instability under certain circumstances. These are issues the factory has always been aware of, and the f/r ride height is set at the factory to minimise these undesirable characteristics.

The body pitch angle is significant as well as the various aerodynamic addenda available (which you already have). It may be that the additional passenger weight in the car combined with your low-ish rear tyre pressures combined to cause a nose-high attitude, increasing frontal lift and causing the stability problems you describe. If you have an extra passenger AND luggage, make sure the luggage goes tightly into the nose, rather than the other rear seat!

Are you running standard ride height f/r? Your car is 20 years old, how old are the dampers? also, how did you derive 29psi f/r? Sounds low at the back to me. All around suspension alignment would be a good precaution.

N.B. - Richard, just for interest, did you fit HD or Sport Bilsteins?