Thanks for your feedback HawkME.
We digged into our investigations alongside with a FANUC technician.
No leak found on J4 or J5. Or no other significant issue.
With a thorough inspection, we noticed « uncapped » holes, perhaps due to tool assisted screwing some peripherals such as a cable carrier with a much too long screw, and some newbie pumping grease, thinking it would be a nice idea the grease the rods …
Robot has now been running full speed for 48hours, No excess disturbance, and all seems fine.
I can call it a closed case for now.
Thanks anyway.