Why does it work only intermittantly?
Mouse refuses to move sometimes.
Make sure that the surface you are running the mouse over is not reflective. make sure the optics aren't blocked. Turn the mouse over, there shouldn't be anything in the way of the "eye".
Use a mouse pad or some surface that is a smooth surface with a solid color. If the problem persists change the battery. Also make sure the dongle is plugged into your computer completely. If you still have problems the mouse may be bad.