I think this is caused by layer styles. If you copy a part of layer, it also copies the style of it. Now if you paste that part, it will calculate the layer styles again, but from the edge of the new image.
Hard to explain, but i hope you get what i mean.
Now the solution:
When i hard copy(all the layers) 8 pix wide and set it as a patter from the left image, i do not get this irritating banding.
How to do this:
select the part that you want to make a pattern off. (the 8 pixel wide and X pixel high part)
Press CTRL + SHIFT + C (this will not copy 1 layer, but all that are vissible and combine it).
Press CTRL + N, then CTRL + V (meaning new file, paste.
Press CTRL + A (select all) and go to edit > define pattern.
Give the pattern a name.
Now fill any selection you want with you pattern and the banding is gone.
Hope this helps you