After upgrading to v6.00 & v7.00 I had constant disconnects and high latency on a PPPoE line.
These new firmwares proved to be highly unstable so I reverted to v5.00 (rock solid with 40Mhz WLAN off).
After upgrading to v6.00 / v7.00, you can't downgrade without doing a special firmware flash procedure.
Details are here: »Zyxel NBG-416N firmware recovery
My tips for this flashing procedure:
To use tftp cmd line in win 7, I had to install it from the Windows features in control panel.
I then let it through windows firewall.
Stupidly; I tried several times without result before realising I was pushing on the wireless pin button and not reset !
Now back to V5.00 and all is good.
The needed recovery files are in the link given. Once you downgrade it's possible to upgrade back to v4 or 5 .