OK, Befote I describe this issue. Here's the setup.. RPi3B+ with external USB HD boot, The PSU is a TracoPower (professional) 5A PSU. The RPi power LED is not blinking nor lightening symbol on screen. HDMI_BOOST=4, HDMI local monitor, keyboard and mouse. VNC connection used occasionally
In 'normal' operation the HDMI screen is solid stable with no interference. and has been running its tasks 24/7 for over a month in this manner.
Right, the issue
When I am also connected remotely by VNC, interference lines appear across the HDMI monitor screen, but not the VNC screen and HDMI screen occasionally blanks for a few seconds, but not the VNC screen. All tasks are still running correctly. The only visual indication is the interference and blanking of the HDMI screen