Hey,
so to explain my problem i will start right from the begining.

I use a AMD RX 480 GPU and a 4k Samsung TV as main screen.

Both are connected via HDMI 2.0 and 3840x2160@60hz works just fine on Windows.
In Linux (At the moment Manjaro Cinnamon) i could not set the resolution to 3840x2160@60hz.

So I tried adding it via xrandr as I thought there seems to be a problem with the screens EDID.
I generated a modeline with cvt which did not worked at all.
Second i tried to generate a modeline with reduced blanking (cvt -r).
This modeline did work.

Unfortunately I ran into a new problem.

In different intervals I get a black screen...
It seems like the screen lose the connection to the PC and directly reconnects.
This results in a black screen for aproximetly 3 seconds and is very annoying.
I dont know any further.
Do you guys have any ideas how I could fix this problem?

Alex

Start with the basics. Try another cable, and make sure it has the quality to support 3840x2160@60. The Windows driver may be specifically tailored to the Samsung or Radeon to overcome some known limitation in either.

Is the trouble the same logging in with a virgin user account? Cinnamon might have saved incompatible settings made with an older display.

Does 3840x2160@30 work? Does 3840x2160@60 work in any other DE or WM, such the lightweight IceWM? What does hwinfo --monitor report?

Manually generated modelines are virtually always unnecessary, diversions stealing your effort better used to find the real problem, especially with newer hardware, such as 4k and GCN4th. If EDID is a problem, preferred mode, sync and refresh specs can be provided via xorg.conf*, allowing X to generate modelines it needs automatically. 3840x2160 should be automatic, no configuration necessary in xorg.conf or Cinnamon.

Pastebin Xorg.0.log from /var/log/ or ~/.local/share/xorg/ from a session that included 3 second blanking so we can see what X is reporting.