troubleshooting

I have 4x, 5810 workstations... manufactured in 2016.  Recently the video display has been intermittently cutting out, then coming back on.... on two of the units.  (Display screen goes black, then spinning dots, then the video display reappears... OR after the spinning dots the screen gets black again and stays black until I reboot.  Power to the computer and screen remains on.)

I've swapped out monitors and components (video card, video cable, PSU, RAM, SSDs.. even checked to see if the mobo batteries were bad) but not... and have run Dell's onboard hardware diagnostic, which reports "no problems".

Any of you experts got any ideas?  Is there an "age related" issue... like with the 5810's mobo going bad?

Thanks.