126 steering wander- 5 year frustration

[S-Class Guru]

Well, I'm about ready to give up.
Any suggestions would be welcomed at this point.
This is kinda long, but it's been a long project.
I've owned this '91 300 SE since new, and it's in flawlwss condition, with 150k miles.

About 5 years ago, I noticed it just didn't have that locked-down feel on the freeway, definitely wanders around a bit.
In the wind, it's downright scary.
First seven years of its life it DID NOT DO THIS.

The only thing non-stock is the wheels/tires.
I went with the 16" 8-hole from Tirerack (with absolutely proper offset).
I didn't notice anything when I installed them, but a couple years ago, I put the stock 15" back on, and the wander still happened.
The MB dealer had checked the car, could find nothing wrong, and accused the larger/wider wheel. That's when I tried the old ones for awhile. No help.

Below is what I have changed over the past few years - to achieve no improvement:

Front:
checked/adjusted wheel bearings.
3 sets of tires - Conti's, Yokos
numerous alignments.
tie rods, radius rod
idler arm busing kits - two
steering damper - two
shocks- Bilstein
lower ball joints (uppers feel like new)
A-arm busings upper and lower.
guide rod bearings
resealed stering box, and adjusted tension on the bench.
(this box has wear compensator - it feels tight and good).
Checked steering coupler, feels tight.

Rear:
subframe bushings and diff mount
spring pads - 10mm taller to correct slight spring sag.
HAVE NOT REPLACED TRAILING ARM BUSHINGS - they seem okay.

Observations:

Steering and suspension feel and drive like new at low speeds and over bumps (they should, they are new).

Car tracks absolutely straight when I release the wheel.

Very little play in steering - 1/4 inch movement in wheel can change the direction of the car at 50 mph. Feels great.

Alignment checks out good, even the non-adjustable rear is in spec. Tire wear is good.

No damage, ever. We did hit a low curb at 15mph once, but saw no damage to wheel or tire, and alignment stayed good.

So, in summary, the car feels and drives like absolutely new around town. Alignment is good, tires are new, All is great.
But at freeway speeds, it wanders around enough to tire me out, and when the wind blows, it becomes dangerous.
Oh, my $15k PT Cruiser does not do this, so I know I'm not just being picky.

I see posts here about MB just being sensitive to road wander and winds, and many folks seem to accept this.
Well, that's NOT true of my 126 when it was newer.
We lived in West Texas, and drove in high winds at 80mph for 5 years, and this thing was like a rock.
So, something is wrong! What am I missing in all my attempts to fix it?

Tks,
S-Class Not-So-Guru