Hi,
I found a solution to my problem, uvw randomizer and c4d variation shader shares this same problem :
those two shader are affected by objects appearing and disappearing that are
on top in the object manager list (including particles).
if you place object with visibility that goes on and off at the end of the list you're fine.
It's not the best solution... hope someone fix this issue.
found my awnser in this post :
https://www.c4dcafe.com/ipb/forums/topic/92435-r17-new-variation-shader-changes-colour-randomly-bug/