Just to be clear, when you say out of registration, you do mean that it doesn't line up with the
other heads, and not just that it's off center?
Check the easy stuff, make sure bolts securing the head are tight, try and physically shift it, etc.
Make sure the screen hanger/holders and locks are tight.
Also check that it's parallel to the platens, you don't have to go crazy here, visually parallel f/b
and side to side is fine.
That it's only one head sounds like an issue with that head and not the registration bearings/forks,
(not familiar with the Sabre but the concept is the same for all machines) but stranger things have happened.