Nexus hub keeps getting misaligned I have a nexus 8-speed internal hub on a Civia Kingfield bike. It keeps getting misaligned -- the dots on the bottom don't line up.

In the past I have successfully adjusted it using the barrel adjuster, and it would work great again for a few months. I attributed the misalignment to cable stretch. However the past two times I did this, it quickly became misaligned again, and now the barrel adjuster is so stiff I can't turn it with my fingers in either direction.

I took it to the shop and they claimed there was dirt in the barrel adjuster and they said they cleaned it out, and they had adjusted it again so the dots lined up. The barrel adjuster turned easily again immediately after they did this. 100 miles later, and it's horribly misaligned again, and the barrel adjuster won't turn again.

Furthermore, the latest misalignment happened quite suddenly to me today. It was riding pretty well, a skip here or there but very rare, and then suddenly became completely terrible; no gear "sticks" any more, and the dots are quite far apart on the bottom.

Any ideas what's up? I am thinking I probably need a new cable and new housing, however when I mentioned that to the shop they said the cable was fine (though I have no idea how they checked).

The bike has about 2000 miles on it and only minor maintenance has been done so far (wheel truing, previously mentioned barrel adjustments, new brake pads, etc.).
It sounds as if you adjusted the barrel adjuster out too far, the last time you adjusted it, and damaged the threads on the adjuster.  When the shop cleans and lubes it, it is fine for a few days, and then gets stuck again, because the threads are cross threaded or stripped.  If the damage is relatively minor, it can act normal until it is under a little higher tension than average, and then "jump" out of place, whereupon it will be stuck, and out of adjustment almost immediately.

Of course, not having it in front of me, this is just conjecture.  You should find a shop you are confident in and explain the multiple issues, and ask them to find the problem.