it does sound like the temperature difference on what is probrably a poorly insulated wall is causing the problem. heating the room will improve matters as well as installing a small low level vent in the outside wall (airbrick or similar). Ultimatly, if the wall is too poor the problem could only be fully resolved by re-lining it or rebuilding in extreme cases.
Treat any damp or damaged patched with mould resitant underseal paint befor any redecoration.
This is a more common problem that you probrably imagine. if you cant solve it with heat and venting, ask a local builder to look at it - most decent jobbing contractors know how to renew a poorly insulated wall by reasonable economic methods.