Mine done exactly the same,it stops when you open the door as a safety feature.It is the suspension air compressor that is on its last legs,if it goes the suspesion will drop to it end stops-not good.
The air compressor costs £235+vat(about£276)and is really simple to change yourself.
First locate it,it is under the car on the left hand side, about 5 inches in roughly between the two doors on the left side,it has a black plastic box cover on it about 12 inches long.
1st undo the two screws to remove the cover,then two retaining bolts so the unit cane be placed on the floor,then remove 4 electrical plugs,then two small compressed air pipes,this will discharge all the compressed air and the car will sit on its arse,so to speak.Finally disconnect the last two larger air pipes and it is done.Fitting is the reverse.Once fully fitted turn on the engine and it will compress up in about 30 seconds.It might pay to replace its air filter sponge which is housed behind the real left light assembly in a plastic snorkel device,this proabably broke your compressor-leeting in moisture/water.
Hint-when removing make a note of plug postions and label the two pipes to get them on the right way round.
I ried to get a picture on this but wouldnt let me.There is also one on e bay new for £150,but no gaurantee.
Good luck
