Dear,

yes, u need internal cleaning. So, see the ball side of the mouse and there the arrows or some sign willl show you how to move in rotation to clean it...mostly it is anticlockwise.. 
open it and then put the ball out..clean the ball too.
When u clean the ball then see inside the ball area and you will find the two nos. of roller inside. one is responsible for horizental movement and the 2nd one is for vertical movement. the will probably have dust rapped around them.. clean it carefully because if you push hard it will be the result of broken clips that hold them.
if you can remove the screw backside will allow you perfect cleaning but keep in mind there are two connecting clips infront of the click buttons that hold the front part of mouse...
do it carefully,
goodluck...

hope it will fix the issue so dont forget to rate it...