Hi Jean,
No problem, I like the rest of the people looking for help over here can obviously see that you're very busy.
I tried your non photon idea, by getting one box to rotate through the press of a button and have another box utilising its variables to rotate and am having the same non smooth rotation issues. So it is indeed not photon.
However, as far as I can see, both are using the same numbers for the rotation degrees, so I don't understand why one would move smooth and the other would move rugged.
I did find out though, that if I have them both rotating utilising real world time (per second) that the main box moves at a constant pace. Whereas, the box that gets all its rotation information from its parent, starts very slow and actually accelerates. Very interesting, but I have no idea as to why nor how to fix it.
Regards,
Simon