Sporadic steering problem

Hi all.

New here.having problem with steering in my 08 superb.

I have replaced all front arms.track rod end's. Antiroll bar bushes. Got car tracked twice.it spot on both times.

When driving steering feels very light and giddy at some times.have jacked up car and can't find any play on either side.checked tyres and pressure 2.5bar all round.there is no leak or noise coming from power steering pump or rack.

225/45/17 tyres all round.

Has anyone else had same problem? 

Is there problems with rack in these superb model.im no mechanic but know my way around cars. And know when something is not right.

Any help anyone?

Thanks Mike.