a) what movement component are you using? (Floating, Character, Vehicle, Custom?)
b) what method are you using to introduce the movement? Add input, change of velocity, are you changing the character location?
c) Are you making changes on the client or server?
You could be setting different values on client and server and then the client might be fighting the information it receives from the server making it jittery.
If it has the character movement then use add input on the character.