I hate to keep asking questions, but did your recent "take up the slack" adjustment affect the wandering problem?
The reason I ask is I have seen many cases where during this procedure the mesh is adjusted too tightly and the steering wheel does not return completely to center. Actually, the front wheels do not return to their castered neutral because there is too much friction in the now overtightened steering gearbox preventing their return. The driver has to compensate and turn the wheel to steer the car straight and the result is a percieved "wander." Hope that makes sense.
If this sounds like your problem, it is easily remedied by a competent tech.
1980 240d 4sp
ASE Master Auto Technician
ASE Advanced Engine Performance