Mouse Jumping Issues In World Editor

I am currently creating a map in world editor but when I use the "Raise Height" tool in Terrain Editor the mouse will flicker back to the exact same spot on the map several times a second. This creates a large bump in the land as the flickers raise the height of the ground each time, and the ground becomes patchy and uneven when I am trying to raise the land on other parts of the map.
Does anybody know how to fix this issue, or have any idea what could be happening?
Cheers.