Those stats are a bit puzzling as well but do point to something very strange going on.
The sync of 3520 after being up 35 minutes, and assuming you took the shot less than an hour before you posted, ties in with the 4064 you posted before if that was an earlier-in-the-day sync, a few hundred variation as described by MrSaffron, and to be expected.
The noise margin of 4dB is probably from a sync-time, "target", SNRM of 6dB and would be normal variation at that time of day. However we are looking for something odd happening, and it could have been the case that your target SNRM is 9dB or even higher and has sunk to 4 in 35 minutes, indicating serious noise variation problems. We really do need the stats immediately after a sync sorry

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There would seem to be either a high base noise or a high target margin or both, as both syncs are slowish for a 52dB attenuation. On a good line such as I have with low base noise and a variation of less than 3dB 24/7 then with a Linksys WAG54GS showing 52dB attenuation I sync'ed in the mid 6000s getting a 5500 profile. Replaced it with a SpeedTouch 585v6 and it showed 49dB and syncs around 7000 with a profile of 6Mb..
Those figures would just about tie in with your previous speeds around 6000. Do you happen to have any stats available from those days for us to examine as well?
Has anything in your house changed with regard to connection to the phone line - new Sky box, phone-connected burglar alarm? Any new extensions fitted or other similar changes? Any new electrical apparatus at all? New phones?
Disconnections during storms?
You haven't just moved to AOL have you? Unlikely I know, but possible.
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