Suggestions about a very weird symptom with CPC 6128...

[ikonsgr]

thanks for the suggestions. i will see to them and let you know.

[ikonsgr]

- Replace 16L8HAL

Is it the original PAL? A replacement PAL could implement logic that causes artifacts (https://www.cpcwiki.eu/index.php/PAL16L8#Fixed_version_.28Gerald.29).

YES! THAT WAS AFTER ALL!! Many thanks to @eto for pointing that!
The chip in fact, was NOT an original Amstrad 40031 PAL chip, but instead a LATTICE GAL16V8D (probably fitted many years ago...), fortunately i have a TL866II programmer, and manage to reprogram it with the new Fixed version (noting: "the Original version from Porchy suffer from a bad handling of the RAMDIS signal. This cause screen artefact when accessing an external extension RAM"= EXACTLY my problem!!!) , after that, no more video noise!   

[McArti0]

Now, since i've already replaced and test (...) 16L8HAL, 

[ikonsgr]

Now, since i've already replaced and test (...) 16L8HAL,

I'm pretty sure that i replaced GAL from another Amstrad 6128, where both GAL chips worked ok. Maybe because the other 6128 was a burned one, where all chips were removed, placed with sockets + new chips, and for some reason, even the old GAL version worked ok with ram expansions on that 6128