Coolrunnings0_2

Hi can anyone help me with this problem I have with my Audi A6 Le Mans 2010, I have an intermittent fault that sometimes doesn't happen for 2-3 days sometimes when turning left you can feel the power steering go lighter on the left and then another time the power steering has gone a bit tighter when stationery left and right. To rectify it I have to switch the car on and off again and immediately the power steering starts working again. I was told that the car has a type of pressure sensor which does allow the car to adjust its power steering which I have now had changed at a cost of £380. 3 days later the same thing starts doing it again, please can someone help me it's driving me nuts.