Check the control arms, make sure they aren't bent, now I don't know how many kms are on the car, but they recommend that you replace the arms every 4 years on average, now if you track the car, every 2 years. But I doubt you'll need to do it that often. Also make sure there isn't any shit inside the rim, (mud sand etc).
Could also be something in the steering as well if everything else checks out.
1990 735iL under construction.....