Well at least I know I am not the only one. My wifes Dell Lattitude C510 has recently been experiencing the same problem. The synaptic pad has been erratic and when we plug an external mouse in we get the same behaviour. rebooting several times seems to stabilise things for a while until it eventually happens again. When her mouse is plugged in (USB) the synaptic pad is still active so I may look at this. Dell have indicated that it maybe the pad on its way out or the keyboard.