Cadence irregular on Zwift

Hi, I just got my Tacx NEO 2T yesterday.

After installing everything, I did a FTP ramp test on Zwift to try it out.

I noticed a strong fluctuation in cadence on ZWIFT, although I was pedalling at a constant rithm, around 85-90 rpm.  

But you can see in the picture below that cadance is often falling down, more so when power increases.  During the cooling down (70 watts) it seemed more accurate, but still a bit jumpy.

I haven't tried it yet with the tacx training app, so I don't know where the problem lies.

Anyone experience the same issue, and better yet, anyone have a solution ?