Mac Pro black screen

So I have a Mac Pro 6.1 (black trashcan one) running Sonoma and I have the problem that I sometimes need to unplug my monitor (thunderbolt) multiple times to finally get an image. Tried different monitors, problem persists. There are different scenarios however. Sometimes when I turn on the computer I see the bootscreen with the white apple, but the screen turns black and says "no video input" after a few sec, resulting in the computer being at the login screen but the screen keeps saying no input. Replugging it (sometimes multiple times) solves it.The other situation is that the screen stays black (saying no video input), not showing the boot screen when I turn on the computer and eventually when it's booted up it shows the login screen out of nowhere.

I have no clue why this happens but it makes it damn hard to get into bootcamp when I need that for work. Holding alt at boot seems to be hit or miss. Especially when the screen just stays black.