Latitude E6420 Cursor Issues

The cursor on the laptop randomly moves to the bottom of the screen.  When I touch the touchpad to gain control, I am able to move the cursor but it continues to move down ward.  This will happen randomly for up to 10 minutes and then it stops.  I have re-installed Windows 7, updated the BIOS and all the drivers including Intel, but this issue still occurs.  Even when I plug in an external mouse, the issue still occurs.  I have also tried disabling the touchpad.  The diagnostic testing has passed. Is still a software or hardware issue?