Hi Jan, I have been having this [relatively new] oscillation issue specifically when using the VNAV and often in LNAV based on the current IXEG v1.33, and maybe since v1.30 onward? Switching to HDG, LVL CHG, and VSI modes tends to smooth everything out. I know this wasn't an issue in prior versions. I am providing two video clip examples here.
I actually planned to report it sooner, but quite often I have been doing my own interventions since I do a lot of online flying with PE/Vatsim ATC, and they often ask me to use other altitudes in the STAR. Since the VNAV T/D has stopped being able to re-calculate since V1.32 (?), I haven't been able to rely on the T/D.
Anyway, today I did happen to let the VNAV do it's thing, and here are two clips that shows what I've been experiencing. The winds were not gusting, nor was there significant turbulence. Perhaps the OP is speaking about this? In this first clip you'll see the pitch command bar, the VSI, and the green vertical predictor on the ND all moving in response to the airframe pitching up & down:
https://www.twitch.tv/torbinator97/clip/UnsightlyAdorableCardChefFrank
Here it is in LNAV. It's a bit subtle in this example sine it was a relative short heading change, but you'll see the roll oscillates about 3°, the roll command also shows left/right movement, and the airplane/horizon movement. If the turn was longer, it would have continued throughout most of the turn.
https://www.twitch.tv/torbinator97/clip/PlausibleTrappedDonutKappa
Hope these help!