Had a customer who had a similar problem, which was eventually tracked down to a sheep having a good rub against the guy-cable supporting a telegraph pole.
Loose connection on the pole, which came into effect when the pole was vibrated by the sheep.
In this case I'd look at replacing all the ADSL filters for a start, just in case theres interference from something else on the phone system
I'd also get on to the ISP and ask them to look at the history - they should be able to gain some kind of idea whats happening