I like to line up the seam side perfectly horizontal or vertical in the uvs and then relax the verts not on that edge. This will usually prevent that from happening.
there's a little button on the right side of the graph editor called "unconstrained drag", that locks the curves vertically. once you click it it works fine.
I don't think that's a seam as much as the end of the piece of wood. It is rather repetitive, though. Try flipping the UVs horizontally or vertically on a few.