Windows 10 screen distortion & flickering with 4K monitor added

My Windows 10 screen often behaves erratically and produces distorted screen, as demonstrated in the attached screen shot.
What's the problem?

(I have a 27" monitor attached to my iMac retina 5k through a mini display port-to-HDMI adapter cable. But I doubt this cable has anything to do with the issue, because it worked fine under Windows 7.)

 

Hey, try to reinstall Parallels Tools and check.
Please provide us with the results.

 

I reinstalled Parallels Tools, but the problem persists, albeit in a different pattern.

 

Try switching DirectX between 9,10 and check how it works.
Shutdown Windows virtual machine.
Open virtual machine configuration > Hardware > Graphics > Advanced Settings > 3D Acceleration to DirectX 9, 10 and disabled.

 

DirectX 9 won't work. It makes Google Chrome keep crashing.
I'm not back to DirectX 10, and the screen problem continues...

 

Please try with DirectX disabled and see how it works.

 

It worked!
But now I see occasional flickering that I didn't experience with DirectX 10 on.
Problem just piles upon problem.

 

Please try reinstalling Parallels Tools: http://bit.ly/1cZWAa3 then observe how it works and keep us posted.

 

I reinstalled Parallels Tools -- for about the 5th time -- but it didn't help.