Connected but no ip

A neighbour of mine has talktalk and I'm trying to see if that's the source of their problems, or if its hardware.
They've gone through three routers now and all of them have had the same problem. At the moment they're using an old Netgear DG834G of mine that always worked fine. What happens on a fairly regular basis is that all internet connection is lost. At first I thought it was the DNS settings, as some pages (google) would work, while others (thinkbroadband) wouldn't.
I applied the OpenDNS settings and this temporarily fixed the problem, however they still have intermittent faults where internet access disappears.
Looking at the router's homepage, sync is maintained to the exchange - about 2meg down and 0.75 up. The ip address isn't assigned though - however a reboot of the router for some reason seems to fix this.

So, should I be pushing them to switch to O2 LLU or is it not talktalk and rather that can I fix this with some new hardware?

From what you say it sounds like the ppp session is being dropped and or a problem with authentication to re establish connection to the Internet, it is possible that it's the router, but likely to be down to the llu dslam at the exchange, my ppp session will randomly reset sometimes without appearing to loose Internet connectivity , but some times it will stay in a disconnected state, requiring user interaction, via the routers web GUI reconnecting it again and i am with a different isp

See this post for the same issue. http://forums.thinkbroadband.com/talktalk/t/3855072-...

PPP sessions being dropped are network related nothing to do with the exchange. Once DSL sync is achieved the exchange is more or less out of the picture. If the MSAN was causing the trouble, it would be very easy to see, as all users connected to it would be affected. This problem is another nationwide issue.