Strange steering wheel shimmy

In prior posts in the TR / BB section I asked some steering rack questions about my 1991 Testarossa. I ended up replacing the rack as I could not source an inner tie rod to replace one with free play in it, in an effort to eliminate a steering shimmy which is felt whenever I reach 140 km/h.

In summary, the shimmy is felt in the steering wheel (not the seat). Its frequency or severity is not affected by being on the throttle or the brake at that speed. It gets worse if I try to go faster than 140.

To try to isolate the issue I have:

Checked all 4 rims for straightness (the left front was slightly bent - fixed now)
Installed 4 new michelins and had them balanced (twice) I typically run 2.5 bar front, 2.8 bar rear.
Replaced a steering rack with a new rack
Had an alignment done (twice)
Cleaned and repacked all 4 CV boots (the rubber boots were split, but the hub bearings seemed fine)
Rebuilt all 4 calipers

All suspension and bushings were replaced on all 4 corners about 6 years and 10,000 km ago (all seem sound, no play or visible damage detected). All 6 Konis rebuild by Koni at that time (no visible leaks). Front steering ball joints look good and show no play when the car is in the air and the wheel is pushed and pulled.

I have even checked to see if any sheet metal was loose under the front of the car which may cause a wind related issue. All seems tight there.

With most wheel related issues I thought I would notice a rhythm in the shake which speeds up as the speed of the car increases. Not this time. The car is as smooth as glass until I hit the 140 barrier.

The obvious choice is limiting it to 139, but what would be the fun in that?

Any ideas?