My trusty 6 year old D800 has sudenly developed a video problem and I suspect it is the graphics card.  There are vertical bands that flicker on the display. I can only get the login window when starting in safe mode.  Am I correct with my tentative diagnosis or might there be another explanation?  The drive is backed up, but I'd like to keep the laptop going if possible; I see I can pick up a graphics card for not too much.

TIA

Andrew