I was trying to identify the circumstances that seemed to make your problem disappear, then reappear, then offer a postulated explanation. MB shocks are a little more complex than some others out there, but it seems your bushing fix was more in line with the problem.
I think you should go back to the last shop, as they seemed to have found at least part of the ongoing problem. I am not familiar with the bushing that got replaced, but it sounds like it is not as straight forward as changing a tire and there could be something wrong again, or with another bushing/connection that was previously improperly adjusted or installed.
Good luck, 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)