no connection - yet there is... what's happening?

[#1]

At the moment, my connection keeps becoming unuseable but not actually going down. My router shows that it is connected, and it's able to resolve domain names but any attempt to go to a website, or even to ping something just times out. It'll stay like that for a while and then a day or two later suddenly start working. My friend, who is also a plusnet user is experiencing exactly the same thing. We're both using the same exchange but don't live particularly close.  I emailed support about this and their reply was simply " ... the problem may be at the local exchange..." which doesn't help me much.
Can anyone throw some light on what might be happening here? And what I might do about it?
All thoughts gratefully considered...

[#2]

The router usually caches DNS requests, so assuming the connection is up on that basis is not a good idea. If pings and access of mainstream websites doesn't work you've either got a firewall config problem or the connection isn't working.
If the sync is dropping, the router may not report the connection as down, as it is capable of re-training to a different sync without "disconnecting" as such. And if it is spending most of its time re-syncing, there will be huge holes in your ability to access the internet without materially affecting your access to the router itself.
What router do you have? Can you get to any line statistics such as noise margin, attenuation, FEC errors, etc?

[#3]

Next time this happens I'd suggest trying a couple of things:-
1. Ping bbc.co.uk from a command prompt
2. Ping 212.58.224.131 from a command prompt
If the second one works but not the first then I'd suggest trying a different pair of DNS servers.
212.159.13.49
212.159.13.50
212.159.6.9
212.159.6.10

[#4]

It might also be worth flushing the PC's DNS cache (also a good idea when you change DNS servers) to see if that has any effect.  To do this you need to open a command prompt and type ipconfig/flushdns - you should get a message saying "Successfully flushed the resolver DNS cache" or something similar.
To open a command prompt go to Start>Run and type cmd in the box and click OK.
To change your DNS servers, assuming you're running XP, go to the Control Panel and open Network Connections.  Right click the icon for your connection (if it's a wired connection it'll be under LANor High Speen Internet, if it's wireless it'll be under Wireless) and select Properties. In the box that follows highlight Internet Protocol TCP/IP and hit the properties button underneath.  Click the button that says "Use the following DNS servers" and enter two of the addresses given in Chris's post above and click on OK.  Close the Properties box by clicking OK and then fluch the DNS cache as above.
(The same holds for Vista but you need to follow the info on this page.)
@cowhead
Apologies if you already knew how to change your DNS etc. but I thought I'd better lay it out in case you didn't!

[#5]

Thanks for the replies.  I'll address the questions etc in order:
@Laser: I can't rule out cacheing, but I can rule out firewall issues because I've disabled them to make sure. I've also swapped router to rule that out. If it happens again, I'll keep that in mind - I'll try and flush, or resolve a site I've never been to, or even factory reset it! The routers in question are both Netgear DG834PN and yes I'm able to check the line stats and they look absolutely fine. I'm not with the router right now so I can't check it but from memory, when this happens, I get plenty of Tx packets but hardly any Rx (?) It seems to me that there is no indication of re-syncing, at least, the speed and noise margin etc aren't changing which I assume they would (?)
@Chris: I've tried that and it makes no difference whether I use name or IP address. My router is set to auto detect the DNS and is picking up 212.159.6.9 and 212.159.6.10 although I have noticed that the support pages suggest I should be using 212.159.13.49 and 212.159.13.50. I wasn't aware of that last ime it happened (Saturday) but next time, I will try substituting the other DNS manually to see if that helps.
@Paul03: I've tried /flushdns and /release /renew etc but nothing has helped.
The fact that the same thing occurs across several computers in my house leads me to believe that the problem isn't a local machine one. Also, because it occurs with more than one router I believe the problem isn't there either, (although I'm slightly less confident about that as by chance they are the same model router, similarly configured.) However, because the setup has worked fine for years and as far as I am aware nothing has changed, I have to lean towards thinking the problem is not with anything in my house. The fact that the same problem is affecting somebody else at the same time seems to back that up... but I'm trying hard to keep an open mind!

[#6]

Hi Cowhead,
I would definitely suggest forcing your router to use specified DNS servers to see if this helps at all.  Quite often it can do.
I've run some tests on your connection and there do not appear to be any other underlying problems.

[#7]

@cowhead
Are you connecting via a wireless connection? If it's the account connected to this username then it doesn't look like you've actually been connected for a while now, I'd suggest visiting this site and checking the IP is the same as the one shown in the member centre.
*edit*
If you can PM me the username I'll check that it's the same account as the one I've been looking at to make sure there isn't anything obviously wrong.

[#8]

More problems today where the connection just stopped working, so I tried all of the things that have been suggested here. Nothing made any difference. I was able to get to the bt test site though...
Then I decided to change the username and password to another users details and guess what - everything started working again! Then I changed it back and it stopped. So it would appear that it has something to do with the user account but just to complicate things a little bit more, the account that I changed it to has also been having the same intermittent problems. Strange.

[#9]

@cowhead
You're not connected via a dial up anywhere are you?
You can't use dial up with your broadband username and password at the same time your broadband is connected. (You will get assigned the same IP address on both connections and things will stop working!)