I don't have a understand why you are having those problems I have never experianced that with any computer I have used an extended desktop with =/ Sorry
i'm not too sure, but fairly certain, its because of the resolution of my second monitor.
when i changed back to a single monitor (the 24 inch) via HDMI and rescaled it to fill the screen.
the desktop grid was fine again, so i think its because of the other monitor and extending my desktop :S
but i dont need it.. so its okay