For a couple months now, since a remote VDSL upgrade in my area of Christchurch, I've been having a problem where my 7490 will get online at a speed of around 50mbit (which is fine) and be okay at that speed for a while, but then (according to the logs on the router) it will adjust to a higher speed of 60-68mbit. Shortly after the adjustment, it turns out that this new speed is unstable and the DSL connection will be completely lost, which restarts the process.

Is there any way to force it to just be happy at the lower, stable speed, instead of constantly trying to increase to an unstable level and then dropping?

All the sliders under DSL -> Line Settings are already tuned all the way to the left (for maximum stability), I can't lower them any further.

Firmware version is 06.84 but the problem also occurred to the same degree with 06.83.