I had the same problem with an older Karrimor pack, because as I aged my spine got a little shorter; I couldn't adjust the shoulder straps so that the upper part of the pack fit snugly against my upper back. There was too much stress on my neck from craning forward.
A new Osprey pack with professional fitting solved the problem. Now I can walk upright comfortably with no neck strain.
Maybe you ought to consider getting a modern, internal frame pack.
