Car shaking after steering rack install

I recently replaced my steering rack on my c230, and took it into a shop for a computer alignment after replacing the steering rack. Everything was fine for about a month and then the car started to shake between 35-60 mph when the steering wheel is straight. It will stop shaking if I turn the steering wheel to the right or to the left. I have raised the car and looking under it, the four bolts holding in the steering rack are still intact and tight. Any ideas as to eat is causing the problem?