Was thinking about this... could also be a bad mouse contact in the scroll wheel. Try a different mouse, or just try unplugging your mouse completely when it start happening again. I'd also see if there's an app that will show you the input signals that your OS is getting from your mouse and keyboard... that will tell you a lot. If there's a signal coming in from either that is causing it to do the action that you're seeing on the screen, then it's a faulty keyboard or mouse. If there isn't, then it might be a faulty OS driver, system config, etc.