Blistering paint....

Hi,

Got a small issue with blistering paint in two different parts of the house. In both cases these are walls that had been wall-papered by previous owners. The underlying plaster was in very good condition, with no sign of any damp issues. One of these walls is external, upstairs bedroom, but the other is an internal wall, well away from the exterior (downstairs hallway, concrete floor). The patch upstair is in a diagonal band, from about 8" wide, rising from 12" above the floor, up to about 4ft high. In a couple of spots the paint has dropped off, and the plaster behind is bone-dry. However there is some chalky residue (efforescence?) on the back-side of the dislodged paint. Neither rooms show any evidence of any damp problems. The problem has occurred very slowly, over a period of about 7-8 years.
I've attached a photo - any dark patches are just bad photography, the paint is a consistent white colour throughout.
Any ideas of causes and suggestions for remedies?

thanks

A