The regulator on a watch, if you look closely functions by moving a pair of pins through which the spring passes, along the outer coil. That makes the hairspring effectively longer or shorter, but only by enough to give it a swing of a +/- a few minutes per 24 hours.
A hairspring can get oil on it, become magnetized, or be out of shape or position to cause this. And you can usually the issue by watching the spring closely. It should expand and contract evenly with each beat, and not "snap" or expand out of round.
On this watch the spring's outer coil had crept upward somehow and was touching, just barely, the bottom of the regulator pins (this is an over-coiled spring, as opposed to flat so the spring does pass directly under the regulator pins). Adjusting the stud down a hair (probably less actually), and re-flattening the spring solved the problem completely.
Why this problem didn't manifest itself before I sent it back to the owner I don't know. The stud was secure, and so is the collet. It seems like no matter long I run watches for before returning them, the occasional mystery still occurs.
Watches can seem frustrating, but they are not mysterious nor magical. They are completely mechanical, governed by very basic principles of geometry and physics. There's always an explanation for a problem.
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