This is probably an issue dealing with the angle of the face normals of your imported mesh.
You can either select the whole mesh and go "Edit Polygons > Normals > Soften / Harden" and adjust the angle option or if your model has many fine and complex details you might wanna try selecting groups of faces at a time and changing the angle of their normals with "Soften / Harden" (different groups of faces might need different normal angles to "distinguish" from each other).