I wrote about this before and may have said more. The real problem is serious. You will undoubtably find the frame cracked. In order to repair it you must understand how it works.
The actual frame is sort of like a 3 by 6 tube of some fairly light gauge steel. Inserted into this tube is a box made of a little heavier gauge sheet and three very heavy tubes. The tubes space the plates so that when the bolts are tightened the channel is not collapsed. The box and tubes are welded together and they are spot welded inside the lighter frame section. By the time the problem exists on the outside the tubes are all broken from the internal box and various cracks occur between the flat sections. The spot welds often are broken. All of these rather fragile pieces form a rigid box when all tied together. Once in pieces rotational loading causes the whole thing to flex and become a parallelagram taking out the new bolts. Ask how I learned all this (bg).
The way I fix the mess is to open a 3in hole in the olutside tube and I weld all the inside components back to one another and to the outside tube often adding boxing strategically (each one is different). I have also repaired this after someone added a 1/4in plate between the original frame position and the steering box. That small amount bound up the box and caused the car to stick any where it was pointed.
This job is probably best done by a welding shop or a metal man in a body shop. Done right it is a major job requiring the steering box removal for starters.
Bosch Master, ASE Master, L1
33 years MB technician