Working on a 2nd hand LS1290 which had inconsistent alignment issues especially the downward alignment from mirror 3 through the lens to the work. Each time the mirror alignment was adjusted it would move out of alignment a few moments later, or after adding the red dot pointer or making any adjustments to the lens housing.
In order to diagnose the issue, the lens housing was fully disassembled, the mirror and mirror mountings removed. At this point it was noticed that the 3 springs providing tension on the mirror mount had become less than springy glued up and stuck with years of gunk and tar.
The springs were replaced and a good alignment was achieved, tests show that the downward alignment drift had been corrected.
Alignment drift on mirror 3 at the head (or any mirror).
Springs not effectively tensioning the mirror adjustment mounting.
Replace springs and test.