I was in the same boat with my 1990 W126. I took it to Sears for an alignment shortly after I purchased it. The car had only 62k on the clock and had never been in an accident. To be short, I'll just say the Sears alignment was a joke. I was embarassed that I even let them touch my car. I then went to the Mercedes dealer and got my $170 alignment and it has been great for nearly a year now.

I had replaced both tie rods, the center link and the steering damper before I brought it in for alignment at Sears. So, they couldn't tell me those parts were the problem.