Here is the solution, you have to correct:
https://forum.unity3d.com/threads/impossible-to-get-perfectly-smooth-motion-with-unity.141417/
Please read and watch the video at the end.
Here another tips:
https://forum.unity3d.com/threads/linear-movement-stuttering.142754/
http://answers.unity3d.com/questions/275016/updatefixedupdate-motion-stutter-not-another-novic.html
At 50Hz both program is so smooooth, at 60 hz none of them.
I hope it can resolve the issue.
thx