Your comment about the 15th of September is about the same as what I have noticed for my connection going wrong. My modem was however screwed, got a new one but still get Vent lag, Disconnects from lag, slow youtube and a whole host of other bad international traffic. After all who uses national for gaming and media really...
15th was about when vent and international gaming went to crap for me as well.
Since then I've bought a new modem and got the account holder to call Orcon about our exchange port being capped at adsl speeds instead of full/full adsl2+ which they have done nothing about. Had this issue before with Telstra and it took them about 2 days to fix, Orcon who knows when.