Hi Again,
The higher your noise margin, the more stable will be your connection and in that respect a high margin's a good thing. But also, the higher your noise margin the lower will be your connection speed, so it's really a trade-off act between speed and stability.
Each time you connect, the BT exchange equipment tells your router what noise margin it should use (the 'target' margin) and the router then chooses a speed that's achievable with the required target margin. Over time, as interference comes and goes, you'll see the noise margin fall and rise, while the conection speed remains constant. If the margin ever gets close to zero, the router will be unable to sustain the connection and so will reconnect at a slower speed, to gain back the noise margin. I'm sure I'm trying to explain what kitz has already done, much better than me, so have a read of Kitz's descriiptions and, in case of doubt, believe Kitz and not me
It sounds like you did reconnect at some point. If that's only once in 25 days then it's probably nothing to worry about. It's difficult to speculate what caused it. It may have been anything from your line being swamped with interference, to the cleaners at the exchange unplugging a DSLAM so they had somewhere to plug in their vacuum cleaner. Only joking (or I hope so anyway)
If you're worried about the reconnects, I would keep an eye on routerstats graphs. But to be honest I'm not that au-fait with routerstats, so I'll leave it to Jeff and the others to help you out on that front...