I have an Audi which had similar symptoms.
In my case, it turned out to be a cracked steering rack, which ran down the power steering fluid every few weeks. It took a number of different opinions from a number of different mechanics to get to the correct root cause (although, it was apparent it was leaking fluid).
Topping it up every few weeks did the trick; but eventually I decided to get it repaired, which has now fixed it for good.
If you do need to top up with steering fluid, just make sure that you use the proper (volkswagen) green fluid.