There are a few of us that have noticed this issue and it might be a calibration issue with CC3 Base Plus and iClone's motions.
I've tried smoothing the animation, that doesn't seem to help at all. Here is a discussion on iClone Users Group https://www.facebook.com/groups/1684100315159672/permalink/2979071078995916/?comment_id=2979334408969583&reply_comment_id=2979409902295367 -video example posted.

I've seen similar jitter with mocap and the wrong body size preset so I think it's simply a matter of calibration when sending it over to iClone maybe both from CC3 and 3DXchange.

One individual believes that this happens when you change ready-made motions. If that is correct it may be a matter of frames being removed that isn't compatible with iClone's Native 60 FPS.
If I adjust the speed in the motion preset, maybe it's taking out too many frames or the wrong sequence. This may be resolved in iClone 8, since you are working towards adding other Native Time base Rates 24, 30 etc. .

But this too doesn't make sense because it's only the legs that are effected.. So back to my original thought I'm thinking it's a joint calibration mismatch issue upon import. I've also tried disabling colliders on the character. It might be both an import issue and speed reduction issue -missing ideal frames.

Instead of having prebuilt animation, perhaps you should make it 100% compatible with procedural animation - a internal system that uses envelopes for movement.. that way it could be compatible with any frame rate & speed adjustment.

Animations could potentially be converted to envelopes and have almost instant auto clean method for jittery animations.

Smoothing frames didn't work for me so this is likely to do with joint/animation mismatch.