up the dmap resolution, and only then start to fiddle with the bias and filter settings. It's basically artifacts.

explanation: it renders fine as a spot because it is illuminating a much smaller area (trhe size of the cone angle). With a directional light it is illuminating everthing from that direction. So the dmap res is split up over a much larger area. By increasing the map res it will increase the average spread of the map's resolution.You can also use light linking to reduce the amount of geometry being illuminated by the light, that will also help out the shadow problem.

Alan