Screen goes black, 4k h.264

[getho]

nested 4k clip on 1080 timeline. Clip has 1 lumetri applied with a mask.

If I try to apply another lumetri effect the program monitor goes black.

This is a general behavior, not specific to this project, But just for giggles I created a new project and imported it.  Once the program monitor goes black it will reliably require a restart of ppro, and often a reboot.

It is driving me mental.  Will a graphics card with more ram help with this?

Windows, Ppro all up to date, GTX 970, 384.94 (this behavior seems driver independent)

I have 3 screen, all different sizes. I must admit when I switch my smaller screen off the situation improves, but is not fixed.

[excited_Genie16B8]

First thing I recommend is dropping down to one screen until everything works correctly.  From there you can add them back one at a time, checking as you go to make sure all is still well.

[getho]

urgh! one screen. Ok I'll try it out