there's paint and paint! Who were your builders and do they have a reputation for using cheap products?
Maybe rising damp. Do you have a french drain dug round the property and filled with pea shingle or the bulg equivalent.
try digging a trench say 18inches wide with the bottom of it sloping away from the wall and the house side of the drain sloping away into the trench.
Fill with pea shingle. OK, this is a fairly basic french drain but could help.
Repaint the wall after having had a scrape down. As an undercoat dilute some tile glue till it's like PVA and paint the wall with this.
It may be worth going to Mr Bricolage for the paint and using the best quality which may also be the most expensive.
Loads of advise from builders and those who think they are builders will no doubt come your way on this forum. Being simple folk ourselves, we'd advise you to keep it simple and take personal recommendation from other folk in your village or locally who can vouch for a builder or tradesman.