Huge Drops in Noise Margin

[#1]

My IP target is set at 6dB and normally when you re-boot the router - a TG582N - it will re-connect at between 6000 and 7000kbps. Which is fine.
In the evenings the noise margin will drop gradually by about 1.5dB which is also what you would expect.
However at completely random intervals the noise margin will crash by about 6dB and when it re-connects it will only do so at between 4000 and 5000kbps with the IP target at 6dB. After a short time - perhaps about an hour - the noise margin recovers to about 12dB.
So the question is, does this indicate a fault in the line and if so how can I get Plusnet to do anything about it? Their position is that between 4000 and 5000kbps line speed is within the target for the line and that I can't expect anything better than that.
Any thoughts?

[#2]

I would not call 1 hour a short time - it could be something electrical putting out a lot of electrical interference in the vicinity being switched on and used for less than an hour, then switched off. If it lasts a short while, it could be a REIN issue: http://www.kitz.co.uk/adsl/rein.htm
A very short burst of interference is called SHINE - single high intensity noise event. I get a huge drop in SNR margin with a large burst of CRC errors occasionally on my line, lasting less than 1 minute, maybe once a day at random times, but I have a different router, which usually manages to hang on and not drop the connection.
It's not necessarily a line fault, but bad or low quality wiring would be more susceptible to picking up interference.

[#3]

Thanks for your information.
I know that whatever is causing the drop is not within my house or my control and my router will hang onto the connection when there is a drop but only if it's a drop from 12dB rather than the 6dB target that it tries to connect at by default.
I have tried other routers but find that the TG holds onto the signal better than most.
So have I just got to lump it?