Rupert Thomas wrote: " I presume the movement of the mouse ball is
tracked by optical
> means, with which direct sunlight would interfer with. My solution
>  to spray the inside of the mouse with black paint, which seemed to help."

Don't spray the inside of your mouse with paint!!!!
The mouse movement is not optical?!
I guarantee that spraying paint in your mouse will ruin it.
Use a cotton tip swab to clean your mouse innards.
Casey Marion