If you do a serch on here for sticky steering you will see it's quite common if the car has been sitting out in the sun.
Easiest thing to try first is to spray some oil around the Uj,s on the steering colum,if that does not work you put a grease nipple in the colum housing to lubricate the bush.[/i
The latest, and it appears the correct, remedy is to very slightly back off the locking wheel that controls the engagement between steering shaft cog and electric motor cog (which is a worm wheel). The theory is that, during assembly, the technician, in rare cases, has tightened this to the tight end of the tolerance which can subsequently cause the sticky steering in very hot conditions, as the necessary slack is no longer present. The age old remedy of introducing lubricant has obviously eased this binding but does not work in all cases and is really treating the symptom, not the cause. The locking wheel is tightened down by two 6mm bolts which go through two machined out slots in the wheel. These slots allow adjustment clockwise and anticlockwise. Apparently the adjustment required is small, so it is wise to mark the original position before slackening off the 6mm bolts. There is plenty written on this fix on the German forums, some of which have photos and illustrations.