I have a similar issue on my Plusnet FTTC service.
The differences are that mine is an intermittent issue that usually lasts for a few days, then more or less disappears & that ISPs are unable to reset anything as DLM in the street cabinet has full control of connection profiles etc.
Basic engineer tests CAN & OFTEN DO report LTOK (Line Tests O.K.) due to the type of test.
Have you noticed an increase in line/signal attenuation that coincides with the start of this issue, or maybe switched to ADSL2+ which uses higher frequencies & is more sensitive to noise "interference"?
What you have described so far sounds like either "something" has massively increased noise levels, or that "something" (like increased attenuation) has massively decreased signal strength.
While connected at the master test socket, using a filter, does SNR alter or your connection drop when using a telephone?
The cause COULD be as simple as one dodgy joint (corrosion/water ingress etc.).
Basic tests will not always detect such an issue as voltage can still pass through, but a something like a TDR test MIGHT detect it.
Another cause COULD be a "bridge tap" that has been added to your cable (think of it as a sort of branch wire).
These tend to cause similar issues.
I have suffered similar issues for many months, the problem being that it is intermittent & has disappeared by the time Plusnet have been able to arrange an engineer's visit.
Your issue, as it appears to be permanently evident, might have a better chance of being located/fixed as it SHOULD be present if/when an engineer is instructed to carry out the "relevant" line tests.
FTTC Engineering enabling/preparation works MIGHT have disturbed a previously good cable/joint.
Would you be able to set up a monitoring program such as RouterStats that would graphically display how your connection performs (or doesn't perform) over a period of time?
Edited by Bald_Eagle1 (Sat 31-Mar-12 08:44:59)