kszaq Thanks for the advice. I have just tested it. Same as before. After some minutes problems as described.
..but only with live hd channels with 50 Hz . Everything else is playing fine!
For example tested some youtube 4k videos last night, shown with 1080 on my tv screen. Amazing.
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edit : although I have a LG Panel (M3702C-BA) I did 2 things and made some further tests :
Installing this version of libreelec: LibreELEC-S912.arm-8.2-devel-20171205200850-r26441-gdba9a67.tar
Added this in advancedsettings.xml :
<cache>
<buffermode>1</buffermode>
<memorysize>139460608</memorysize>
<readfactor>20</readfactor>
</cache>
As long as I don´t zap from one Full HD channel to another one directly - the picture is fine without issues.
Zapping directly will bring back the problem.
edit 2 : after 3 h without errors the same bug again
went back to to android for now