Nissan Maxima power steering problem

Hi I'm Leo, I have placed 215/55 16 tires/alloy on my Nissan Maxima 1998. Since then, I have a serious problem with my steering wheel. It gets jammed and very hard to steer while I'm driving as if the oil pump is not working. My mechanic checked it and says the oil pump is working perfectly. The problem could be related to axcels. Can you figure out what is wrong with my power steering? The funny matter is when I take the car for a car-wash station, the steering wheel becomes soft and easy to steer but not more than 15 minutes.