Citroen Xantia suspension hard and 'wobbles.'?

Over the last six months, the suspension on my T regd Xantia 1.9TD Sensation has become hard and when I go over a sleeping policeman,for example, the car jiggles slightly from side to side.I've just had it in for a MOT and the garage said it may need it's spheres changing,or it may be a valve that's stuck.The fluid is up to the correct level and the ride height is set correctly.Otherwise,cracking car!