Bump, here comes a quick update about the data wheel crazyness problem.
I changed the flat flex ribbon cable going from the control board to the mainboard and the startup problems disappeared.
The wheel still has some erratic behavior while scrolling clockwise, but far less than before.
Anybody has any idea about what could cause this kind of problems?
Knowing that the control board PCB and the encoder are new (changed it twice, believing at first that there was a stock issue).
I don't want to change the whole motherboard, I think there might be some bad transistor somewhere but I definitely can't trace the circuit myself. (I could provide the service manual on demand, though, else it's everywhere over the internets)