Hi Evo, the flaking is caused by one of two things Dampness or that the plaster still has alkali init this alkali re-acts with paint and in time will cause it to peel off , firstly wash the plaster with a week solution of viniger and water this will kill the alkali in the plaster then coat the wall with a PVA sealer then when its dry it should be ok to paint over without any peeling hope this helps
jim, January 2010