Same recommendation: check your phone line SNR and Attenuation like talked about earlier in this post. If your phone line is bad/funky, you could change your modem every hour/everyday and it wouldn't make a difference.

As mentioned above, it could be a bad line and not your modem.

+1

Loss of signal is usually caused by bad internal wiring or a fault on the line.

Make sure your phone(s) are properly filtered. Do keep in mind, a faulty splitter may also cause dropouts and lower the overall quality of the connection.

as stated the modem/router restarts/reboots itself. So how could it not be the modem?

I was thinking re-sync rather than reboot. If the device restarts on its own, I would narrow it down to being one of two things:

1. Faulty modem (like you said);

2. Your line was upgraded to ADSL2/2+, in which case a firmware upgrade will be needed. Check with your ISP.