Ah, I replied to the post before the one saying you rebooted, and hadn't seen the one after it.
The noise margin is always reset at a re-sync/reboot to the level determined in the DLM. It then varies all the time depending on the actual noise. If noise goes up the margin falls, and vice versa. Which is why daylight sync's are normally faster, as there is less noise.
The figure immediately after it sync's is worth knowing, as that tells us the DLM setting, which us the one to worry about. It used to default to 6dB, but these days on a clean line it sets to 3dB.
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