It wasn't stated in your stats which figures were upstream and which were downstream, so I'll assume that the first figures are upstream. You have very low attenuation and respectable noise margins, so one would normally expect a pretty stable connection. Something is producing serious intermittent levels of interference. Possibilities are:-
- poor internal extension wiring picking up unsuppressed interference from an electrical appliance
- a very bad local source of interference (exercise treadmills have been implicated a few times)
- an intermittent line fault
You can eliminate the first item (and in part the second) by plugging the router into the test socket behind the master socket faceplate (assuming that you have an NTE5 master socket).
As to the third possibility, do you hear any crackling on the telephone, and does using the telephone affect the ADSL connection?
And finally, you do have every piece of telephone equipment (including Sky boxes, fax machines etc.) connected via a filter, don't you?
Eric