I think there must be water running from somewhere, it could be a pipe in the wall corroded, the groun too high outside or a leaky gutter . If the plaster has been damp for a long time, it may have perrished in which case it could be a case of knocking off the effected area, letting it dry, sealing the wall and re-plastering. If the ouside of the wall is pourus and the water is coming through it would be then trapped between the plaster and the inside wall with no means of escape. If this is the case, you will have to seal the outside of the wall as well.