Yes that all sounds familiar. I have to drive fast officer, its the car you see...
Mine doesn't do it to the point where it worries me, I am relaxed and loose with it, but it will "tramline" like a good un.
Tyre pressures... Doubt this will make any difference, I run 2.7/2.5, F/R in the Alps.
Also had the pinch bolt on the pitman arm clamp loose. The arm was moving on the splines, only very slightly, virtually undetectable to the eye, but a few thou multiplied by the length of the arm becomes significant. It was noticeably better when retightened, still tramlined, but tighter in the steering.
I actually replaced the OEM nut, which is a sort of half nut with a full ss nylock.
Do you have powerflex thrust arm bushes? I am on my second set after similar mileage to you. EDIT: No you don't, just read your post properly.
gix