Granturismo Sport steering vibrations

[alfacorse]

I have some vibrations on the steering in the speed range between 80 and 110km/h. Seems like a wheel not well balanced, so I had all the 4 wheels being re-balanced from my tyre shop but this didn't solve the problem.
I was thinking to a defective tyre, but then I've switched to a new set of rims+tyres for the winter and the vibrations problem incredibly didn't solve, so it seems not to be related to wheels or tyres.
What else could be?

 

[safrane]

Warped brake disc could be a reason...does it increase when you use the brakes?

 

[alfacorse]

nothing related to the brakes. It's not changing when braking or not.
What changes is that when I start from home the vibration is at the maximum, then after 15-20km decreases. Could be that tyres are getting flat when the car is parked, but how could have the same behaviour with 2 different set of tyres (winter 19" and summer 20")?

I feel the vibration at 110km/h, then it disappears completely and comes back at around 210-220km/h, to disappear again after 230

 

[safrane]

I get similar on my GS but I only use it once a month. You can over inflate the tyres if like me the car sits for a long time...and yes the cold will make this worse as the tyre takes longer to warm up.