My 98 E300 has the same problem. I flushed the power steering system with fresh fluid last summer, and that helped for a while (the stuff that came out was black). As the weather got cooler, it started acting up again, so I flushed it again and replaced the fluid with the Pentosin synthetic power steering fluid used in the newer Mercedes models. The dealer recommended the synthetic fluid as something worth trying before buying a new rack. They have had some success using the synthetic fluid to solve W210 steering problems. He said a sticking orbital valve is the problem. My steering is stiff sometimes after the car has been parked for a while, but it is rarely stiff in warm weather. I have also found turning to the left before turning right frees the valve most of the time.