moving mouse locks screen

Hi,

I've had an issue thats been driving me mad for over 3 months, and can't seem to find anything on the web... This is a iMac 24", latest OSX but has happened with the previous OSX and "it" is that if the mouse is moved in a quick diagonal way across the screen... the screensaver starts. it just has to travel a few cm across the monitor and its instant screen lock.

It becomes very annoying when you move your hand to the mouse and that causes the mouse to move subtly, that often kicks it in.

thanks!