When the Vigor120 is connected without PPPoE bridging, browsing is fast and reliable. As soon as I enable PPPoE bridge mode, and connect it to my router (either a custom FreeBSD install, or pfSense), my Internet access becomes slow and unreliable. The Windows PPPoE client seems to work okay with the router in this configuration.
Browsing to sites such as speedtest.net, most of the page loads slowly, but Firefox eventually pops up with a message saying "The connection was reset". General browsing is also very slow.
I have investigated setting the MTU and MRU settings in ppp.conf as low as 1400 (as suggested by DrayTek support), without success.
There have been numerous reports of people using pfSense with this router (including on Geekzone), and no other reported reliability issues as far as I can see. My custom FreeBSD install was working fine with Compass Wireless. Suggestions on what to investigate would be appreciated.