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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

Ideas? thanks

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It looks like the GPU can't work fast enough for both display and VNC capture. To help this poor GPU, we can reduce the display framerate by adding :

dispmanx_offline=1

to the file /boot/config.txt and reboot. This will enable the double-buffering method.

There will be less refresh rate on your screen, but every frame should be correct.

Edit : you can also try another memory split, giving more memory to the GPU and less for the RAM. Add or edit :

gpu_mem=128

in your /boot/config.txt file.

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