Hi Petronic. You have a curious problem here! The only real issue i had with my steering was a leak and when the oil got low it behaved oddly, and got quite dangerous. The only other stuff around the steering that wears out is the track rod ends (3-5 years) and the bottom ball joints, same 3-5 years. the rack gaiters also get perished and need replaced, and these can cause problems. The hydraulic unions on the rack itself can come loose if inexpertly worked on (my initial leak was these unions loose, i got new copper washers and it was fixed. your problem sounds to be related to the power steering itself, it unlikely in my mind to be ball joints or track rod ends as these would not give you a center position problem, they would just give you vague steering probably.
How stiff is the steering? are there any noises associated with this? I would think if the pump belt is slipping, or indeed if the power steering is working against a stiff UJ or the steering wheel is binding some how, it would get noisy where its working hard? If its a lack of hydraulic power it seems odd that its only in the center position? if the pump is not working you would expect it just to be hard to steer no matter what position its in, When i lost fluid (so no hydraulic power) the steering kind of locked up on me, and it was near impossible to move, not subtle and not position dependent and quite scary. Not sure any of this helps but you need to either get in there and start looking for trouble or has been suggested get a garage that knows these cars to have a look?