perplexing driver and black screen problem

Ok, Ive been round and round with the developers on this issue now and
time to turn here. I am running a program called Affinity Photo on a
Dell XPS 420 with 8 GB Ram. I am using both Win 10 and 7 (dual boot
with either OS chosen at startup).

The problem I'm having is that within an hour or so fairly heavy use of
the program, I get a sudden black screen with no cursor... nothing...
and the only way I can get everything back is to reboot. The developers
are quite active when it comes to resolving issues and I have sent them
crash reports, but I don't always get such reports with the black screen
issue, only if I am getting exception errors.

They keep telling me that it looks like some incompatibility between my
video card driver and their program, but say the driver is at fault and
not the program. My big issue with this is that I have tried using two
separate video cards, with former card uninstalled and latest drivers
reinstalled per respective card, and the black screens still occur no
matter the card.

The two cards I have tried are the ATI Radeon HD 3870 and the Ge Force
210. I get black screens with each card within an hour or two after
running heavy image editing/processing operations in Affinity. I get no
black screening with any video intensive games, nor any other
application, only Affinity, yet Affinity says it's likely the video
driver (but both drivers?).

If anyone has some ideas, I would sure welcome them. As I say, mostly
black screen in Win 10 but sometimes 7 as well. Latest video drivers
used for each card.

Been trying to nail this down for over a month now. From the beginning
after Affinity install have I had the black screening. I am trying beta
versions, same thing. I have been running Win in clean boot mode, same
thing.

Thanks in advance for any help or suggestions!