display shuts off at random

[Jim]

I have a dinosaur running Kubuntu 18.04 with a Gigabyte P35-DS3L 
motherboard and an Intel Core 2 duo E6850 CPU and 8GB RAM.  For a video 
card, I have a GeForce GT 520.   A few days ago it started acting up.  
Now, at random, it acts as if the video card shuts off.  Both monitors 
give a no signal message.  However, the software I had running continues 
running.  I know this because the dinosaur still plays the audio.  The 
keyboard doesn't respond. crtl+alt+backspace doesn't work.  I can ssh 
into it, but it won't reboot.  The only thing I can do to recover is to 
hold down the power switch until it shuts off, then turn it on again.

Any idea what this might be?

thanks