Direto - oscillating resistance

[Denver]

I just unboxed my Direto and have completed my first ride but this wasn't a pleasant one: I think there is an issue with my Direto.

The issue is very noticable when driving on an uphill Zwift section with a stable slope (eg constant 4%) with lower intensity (around 150 watt)
No Zwift ERG training, just a free ride on Zwift

In this case the resistance feels to be oscillating between no resistance and very high resistance every few seconds instead of a stable normal resistance for whole the time .. this makes the resistance feeling very unpleasant and the roller almost impossible to use as you constantly change between to easy and a wall feeling

I've done a new calibration, which showed a value of 6406 (below the offset value on my roller which is 6409 )
As it felt like a slipping belt, I followed the steps in the manual to increase the belt tension, after this I calbrated again, showing 6409
However this didn't solve the problem, it's still an oscillating resistance.
I also played with the trainer difficulty setting on Zwift between 40 to 100% but this also didn't help at allsnaptube.cam/ syncnet.onl/telegram-web/ 9apps

Anyone has the same issue?
What is the solution for this ? Or is this just a faulty unit which I'll need to return?