I have an old 3-gang metal junction box that is nailed to a stud on one side, via the "new work" straps above and below it. No other part of it is fixed to anything, and it has become a little loose over the years (or perhaps it always was) so the opposite side rocks in and out of the wall slightly and, at rest, is a bit recessed into the wall.
The plaster wall on which it's mounted is not in the best shape, and not very straight, so the devices' plaster ears don't do their job very well.
Is there a reasonable approach to improving the strength and alignment of this box other than reinstalling it? The objective is to provide a better foundation for devices and wall plates so they don't move and flex while in use, but without breaking open the wall to install a new box (IE avoid plaster and paint repairs).
The relevance of "3 gangs" is the degree of movement in the side furthest from the mounting stud. I have this problem is smaller boxes too, to a lesser degree.
See this question for the consequences of not repairing this: The electrical devices installed in the box are easily damaged by rough treatment.