However, what would be the remedy for this? Raising the nut? I'd not want to have the raise the action significantly.
It could be a problem with the nut or a fret problem or a relief problem.
Its fairly straightforward to check the nut action. (viewtopic.php?f=11&t=64845#topic4
) If it looks low then perhaps just shim the nut temporarily. A couple of pieces of ordinary printer paper will be about 0.2mm and might be enough to totally cure a problem, if it is the nut. If that sorts it out perhaps get a new nut made. Small changes in nut action are barely detectable in the left hand.
Changes in humidity and string tension might also be enough to alter the neck relief, but that's harder to measure.
I don't think its a fret problem, but its always possible. You would need a fret rocker to check individual frets and a proper straight edge to check for relief, (Its an odd length) so simpler to leave that to a luthier, if necessary.
"When I was younger, I could remember almost everything, whether it happened or not." Mark Twain