The display on my 640M goes blank at random intervals after boot up. Could be 15 minutes, could be 2 hours. When I hook up an external display, works fine with no issues. When I try to toggle between external monitor and LCD, message displays stating "No External Display is attached", but still only shows display on external monitor. Have replaced inverter and LCD. Double checked all cables for secure fit. Could it be the flex cable from the system board to the LCD?