Believe it or not, getting a new mousepad could fix your mouse sensitivity. Alternatively, use your computer on a rougher surface to ensure it will not interfere with the laser sensor.

Mouse jitters in League of Legends can be caused by dust on the mouse sensor or mouse pad, as well as faulty wireless transmitter/receiver and USB Port.

Dirt could be covering the infrared sensor under your mouse. There could also be dirt inside the scroll wheel, in which case you’ll have to open the mouse with a screwdriver and use canned air ...