This happens to my Metro - exactly as you describe. I think it's just very sensitive to magnetism. Even a small amount seems to cause small (+5 to +15spd) drifts, which is unusual since most of my watches don't show any effects until they are really magnetized and drifting +30spd or greater. I also find the watch will gradually recover itself if the magnetism isn't too strong.. quite how you describe with going from +15spd to more normal figures without additional demagnetization.
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