Thanks.
Here is the setup. I have two machines, an HP and a DELL, and two monitors. One monitor is always connected to DELL, the second switches between DELL (extended desktop) and HP.
Awhile back, I was not able to find a reliable KVM switch and came up with a "poor man's solution". I bought a KM switch, which works flawlessly. The monitor is switched manually, i.e. it is plugged/unplugged between the machines. However, it is not plugged into the machines directly, but via extension DP cables (~3ft), which are attached to the edge of the desk (so I do not have to dive into the corner and look for a port).
The monitor *never* goes black when plugged into the DELL extension cable, so I did suspect the HP extension cable. However, it goes black sometimes when no mechanical vibrations occur, not even typing. That's why I started thinking of the display or MB.
Should I buy a different DP extension cable first?
Thank you!