The whole point of the ADSL Max product is to get as high a data rate as possible by pushing the noise margin. 6dB or below isn't necessarily bad, indeed some ISPs target 6dB margin for their customers.
Don't believe everything you read on Wikipedia.
You do, however, have an issue on your line somewhere, it is picking up a lot of noise from outside. During the evening when people get home, switch lights and appliances on, etc, it increases the ambient 'noise' on the frequencies ADSL uses which is causing your problems.
I wonder, have you only recently started using your heating? Something as silly as a boiler clicking on can cause problems on a long line
