isochronism

On another watch forum, someone has an old pinlever watch that runs OK
for the first 12 hours or so, but looses 5 minutes in the next 12.
Several have suggested that the mainspring has taken a set, but that
doesn't sound right to me--If it were weak, wouldn't the watch be more
likely to gain, and wouldn't it refuse to run the whole time? I'm more
interested in the theory than fixing the specific problem, although
both would be welcome.

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