I have observed some of the sequence of events you have described regarding the issues you have been experiencing with the modem.
When I lose a connection, especially at night, around 7pm onwards. I can not get it back, no matter what I try, until the next morning.
When it drops out I try turning everything off, isolating the modem and then turning just the modem on and then the computer on. Sometimes this works for a short time. Other times the ADSL light just flashes, sometimes the ADSL light is on, but the PPP light won't come on, and finally sometimes ADSL and PPP light is on but it doesn't work. I have also tried resetting it and that again only works temporarily till the next time it drops out.
There are a few specific option I would like for you to investigate regarding the issues described.
1) Change the modulation for the NB5 (show advanced settings > Setup > modem) to "Multimode". You will find considering your are on an ADSL2+ plan, this would assist in negotiating sync more effectively on your connection.
2) If you are using the Billion modem on the same service during sync loss could you check on the modem statistics to see if there is any variation on the connection?
3) Netcomm are also carrying an NCT11 firmware developed to address some dropout issues the NCT6 wasn't able to. This firmware is scheduled for public release within the next few days. You might check with email@example.com
directly for an advanced copy to be emailed to you.