[speed220mph]

Peter: The problem with your car isn't shock valving since what you described occurs during a steady-state situation. Sounds like your car is running with a "bad attitude", or the front is higher than the rear. Air is packing under the car, causing the front to lighten up, increasingly so at higher speeds, causing severe understeer. When you run over rough surfaces, the front seems to be "ratcheting down" due to excessrive shock-absorber jounce control.

I hestitate to give you a quickie fix suggestion, but I have an appointment to meet. But here goes: Set the front end ride height lower and dial out some REBOUND control. I hope your shocks are adjustable as is the ride height. Give that a go and report back. No use spending big $$$ for shocks and find the shocks weren't the problem. Use the KISS (keep it simple stupid) principle first and see what happens.