This has been happening on and off for a year or more now, the assistance level seems to reduce at random. It is more common in cold/ damp conditions, it does not make the car un-drivable but roughly doubles the effort needed to turn the wheel. The steering will not self centre very well when the assistance is less. As far as I am aware there is no option to set the steering weight on this model so it's not jumping between different settings ? The car is due a service end of the month so I can inform garage but as it does not do it all the time would it appear as a fault code. Has anybody come across this before ?         

I think its electric steering on the Superb and it sounds like there is an issue with it. I would definatley mention it.

Search through the forum. A software update for the power steering was mentioned not do long ago.