To be clear, are we talking about the visible horizontal bands or the vertical seam? Because if it's the horizontal bands, upping the lightmap resolution (by lowering the number) should have at least produced a visible result even if it was just higher resolution banding. But if it's the seam...
Did you make sure the faces' textures are aligned? I'm guessing you just selected the faces and applied the texture, which defaults to world alignment—this makes the texture stretch more as the angle approaches 45 degrees, flips it on north- and west-facing sides, and creates seams at the northwest and southeast "corners" if you're making a cylinder or other curve.
If this is two walls that have a rounded corner, I recommend using the alt+mouse2 trick: Select one of the two flat walls the corner is joining, then alt+mouse2 on the next face over. Then select that face, alt+mouse2 on the next one, repeat until you've hit the other flat wall.