resistance(load) fluctuations issue

[gizmo75]

I have trouble with resistance(load) fluctuations when using a virtual riding program.(eg. myEtraining, Rouvy, RGT …)

Here is my trouble in detail.
Purchased in Nov.2019~ No problem until Jan.2020.
The resistance(load) of Direto is going up and down when riding a 2-16% slope.
Instead of a virtual riding program, I had tested it in the manual (level mode of myEtraining App)
*Level: 1-5
I can’t feel load

*Level: 6 ~ 10
I can feel a bit load
And there is no up-down phenomenon of resistance when riding over 200W-power.

*Level: 11 ~ 16
I can feel a big load with unique resistance’s sound,
But, soon the resistance disappears and then reappears. (load up & down)

[My Test condition]
Direto <-> iPhone 6s (iOS 13): bluetooth: using myEtraining App
Direto <-> MacBook (MacOS 10.14.6): using myEtraining App

Please, Let me know your solution about Direto Load up-down issue.
To solve my trouble, Is It might upgrade or downgrade the current firmware version(57) ?