Dell M6700 Screen flickers and goes white

I have been trying to determine the cause and correction for a Dell Precision M6700 laptop with an AMD FirePro M6000 video card.  If the system is turned off for a significant period of time (30 minutes or longer) the system boots and can see the LCD screen fine--articulate it back and forth, no problem.  About five minutes into use, it begins to flicker solid white before going and staying white.  Occasionally it will flicker normal, but then white again.

If I reboot, the screen stays white throughout the boot process.  If I connect an external monitor and toggle between the video output, I can see the screen on the external monitor.  I can do this even while booting and the screen is white, and see the boot process on the external monitor.

I have replaced the internal video cable (one site recommended) as well as the LCD panel (also recommended).  Short of replacing the video card or motherboard, what else can be done?  Given I can see the video on an external monitor, I am convinced changing the video card will correct the problem.  Given this works fine and at, I am wondering if heat related.  I made sure the fans were clear and heat sinks were both firmly tightened.