I would also suspect the pump is having a hard time keeping up at low rpm if you have not changed the fluid and filter in the power steering system in a while.
With a filter element clogged, the suction supply to the pump is choked off and the pump will cavitate. Once the pump starts to cavitate the power steering system loses the pressure supply it needs to provide the power assist. And you get a kind of chunky response when you turn the wheel. When the pump builds some pressure, the wheel responds as you expect, and then, once you move the wheel some, the pressure fades as the pump is starved for fluid on the input. And the assist dies, the resistance goes up and the wheel feels like it is freezing, so you stop moving it. Then the pressure builds and the sequence repeats itself.
Changing the fluid can be a messy job, and I have tried sucking it out of the reservoir using a chicken basting bulb, and disconnecting the return line from the power steering box and doing a fill and drain operation with the car running. The first way is the easiest, as you just draw it all out, change the filter element, fill it with new fluid, and run the car for a minute or two. Then you repeat the process (minus the filter element) until the fluid is clean and clear, like it comes out of the bottle. Takes a lot of fluid but it works.
The other way depends on the set up of your pump and I am not familiar with your model car. But the idea is to use the bulb to lower the reservoir level, then remove the return line from the steering boost cylinder and have it drain into a coffee can or the like that is about 2 quarts or so. Change the filter element, and have someone start the car. Add fluid as the level drops, and when the fluid going into the can is clean and clear like the stuff coming out of the bottle, shut the car off. Put the line back into the reservoir, top off the reservoir and close the unit up.
Hope this was not too long and helps. Jim
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)