I've just recently acquired an old XJ8 and a lovely old thing it is too, but..
It has 255/45/18s all round and the tramlining is bad, tread on all 4 are 4-5 mk3 and at the right pressure. My partner and I were out in it yesterday, she felt it too.

Now I know people put these sizes on and have no issues at all so what's wrong with mine?. I feel want to go down to 225/60/16.

 

Not a lot happens here at all going by other posts