RE: Help Harness lines
Very curious indeed. Based on your original post, you indicated that the webbing was fine and could be reused. Are the style of lines you are using the kind where you have to tie on the line to the webbing? If so, have you tried the basic fixed lines where either end of the line is actually sown right to the webbing? The basic sown fixed lines are less apt to allow in sand, and there's less chance of movement between the line and the plastic tubing.
Roger's suggestion of switching to a reactor bar type design just might be your salvation here. I actually prefer the SS hook design, because the roller assembly would evidently wear out, but you might just find the roller hook really works best for you.
If all else fails, you might want to consider investigating using SS cable inside the plastic tubing. It's usually available at most marine chandlerys, like West Marine or Boater's World. While I've never seen anyone use SS cable for harness lines, I think a lightweight small diameter cable should have enough flexibility and strength to be workable, and there are plenty of different types of marine SS components to successfully terminate either end.