So,

Are you asking about replacing it or adjusting it some more?

Not too bad of a job, if you've done tie rods and other parts swapping out the P/S box is just a few more bolts. I would maybe try and tighten it up a bit more, just for grins, before I dropped the bucks on another one. There is a procedure for doing this, wheels must be straight ahead, some sort of alignment pin in the box specially designed for this but I have never used it. Remember, counterclockwise on the center adjustment stud will tighten the box. Test drive it and be sure it does not stick right or left, it should self-center itself somewhat.

I am sure there are more experienced folks here that may chime in soon.

BTW, I know where you are coming from, I had an 79 450SL that suffered from the same problem. Car was just plain squirrelly, didn't matter what I did, could never get it totally tightened up and ended up selling it for partially that exact reason.