[Misc] Improving registration ?

[crumbcabin]

I've noticed when printing 2 or even 3 colours for a poster for example, I can get the registration aligned on only one half of the page , then the image ever so slightly moves at an angle towards the end of the page.
Does anyone know of a way I can straighten the masters made on my drums? Perhaps in test mode?
My machine is a RP3105 and I own the technicians manual if anyone knows of a specific page that might help?

Thank you !