Hi
open the http://ip.address.of.netgear/mode.htm you can then put in it modem only mode for pppoe

If it's the LAN port failing (not the ADSL/WAN port) then it might not be obvious from
the network. Try connecting the modem directly to a PC to compare.

We've had a few v120's do exactly the same thing too!

I suspect theres a dodgy batch going around because its rare that we see so many drop and reconnect every few seconds
Must have seen about a dozen start doing this in the last month or so!