It shouldn't do that - that's why it has a regulator - and there's an additional valve in the FDV (under the plug with the locking ring on the side of the thing)
Is the accumulator sound? To test the accumulator set the car to normal height and let it idle for a couple of minutes. turn off the engine and sit in the boot - the car should sink a long way and rise again after about 30 seconds to its original position - just as it does with the engine running.
If it doesn't rise and the system can be heard cycling at about 10 second intervals (often short pulses of noise) the accumulator sphere must be changed.
The valve on the FDV needs to be set with a pressure gauge - but as a rough check the ones I've seen the plug is about level with the surrounding surface. If its a used one its possible someone has played with it. The ring with 2 slots in it surrounding the plug are a locker and must be slackened before the plug itself can be turned (with a hex key I think)
