CenturyLink DSL connection problems

I’m having an interesting time on CenturyLink ADSL. Currently I’m on their “legacy tier” network which is sold as a 12Mbps connection, but when I saturate it with a fast download it’s been dropping. Their 2nd tier support told me they have a line monitoring system that automagically adjusts the connection to reduce overhead and that may be the culprit.

My setup is a Netgear DGN3500 running OpenWrt Chaos Calmer 15.05. I’ve tried a few different modem firmware files and it hasn’t made any difference with this issue.

In the system log on the modem with debugging enabled in pppd, there is no clue as to why the session is dropped, it simply says modem hangup.