Bike bumping mysteriously on smooth surface

I have a bicycle (rear wheel drive) that starts bumping up and down rapidly when it exceeds a certain speed (around 40km/h). The problem is not power related, when I drive down a steep hill with little power, the same problem happens.

In the real world one would adjust the damper, spring and wheel damping, but here there is no effect.
Nor does adjusting mass, slip and drag solve the problem.
I also lined out exactly the front and rear wheel collider however without any difference.

Is anyone familiar with this problem and knows a solution ?

thanks a lot.