How about the idler arm bushings? Very common problem. That would do it.
Any of the many balljoints in the steering linkage that were not replaced would also be suspect.

Also, how much play is in the steering before the steering arm starts to move?
Sometimes play in steering box or the steering wheel shaft linkage can cause for rough going on windy days.
This is also very common.

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1986 300E 5-Speed 240k mi.