sudden stiff steering but ok when restart car

[AaronHill]

hi there my name is Aaron and i am new to this club i have just bought a 2009 a5 3.0 tdi quattro yesterday 

before buying car a took for a test drive and found it was ok intill after about 20 min the steering when stiff and had to force the wheel to turn the garage agreed to fix every thing before delivery they did they said they put new steering rack in to fix the issuse 

i received my car yesterday and all seemed good for about an hour and all of a sudden steering went stiff again after going over a speed bump was only going about 10mph but when turning the engine off and restarting the car everything is ok again in till it did it again on a flat run 

dose anyone have any advise or answers to why this is happening