There are a couple of potential issues with any 'delta' platform suspension, the rubbers can go funny and such, but my first test would be to swap front for rear wheels and check the tyres thoroughly for any bulges or crumbling tread patterns.
Do you still have original Michelins? Are they the ones that came with the car (tell us what the year code is)? If you have the original fit tyres then swap them immediately You might have been nicely warned of an impending high speed tyre failure.
I just had almost the same on my car.
A slow oscillating of the steering wheel between 20 and 35 mph.
It gradually got worse. And then I could feel a vibration through the floor.
Last Saturday I took it to my local tyer specialist.
The offside front tier had a bulge on the inside and it was not round.
Simple solution.... Replace tire.
The standard tiers had a well documented fault.
They are poor quality and dangerous.