..... fault never seen before??? please help

[#1]

We have broadband at two properties, for the past couple of months we have suffered connection issues at both properties.....
on all ocassions the following symptoms exist.....
router sync - yes
ppp - yes
ip address - yes
dns - yes
ip ping - yes
internet browsing - no
the problem is sometimes resolved by swapping internet username & password on the router for the one we use on the 2nd account
The problem is always resolved by swapping out the router for another router (but if swapped back to the first router, this also works)
Note: setting dns manually did not resolve fault
I'm an IT Engineer and have resolved many broadband connection issues but haven't come across this one before
......has anyone else had this problem?
would be very grateful of any help offered, as I could do without having to have two routers at each property
Thanks
Kev

[#2]

Are you running any software firewalls (including the Windows firewall) on the machines? 
If you are, try disabling them temporarily to see if the problem is resolved.  Also make sure that your local network is in the "trusted zone" (or similar dependent on the firewall you're using) otherwise all your internet traffic will likely be blocked.
I can't think of anything else that would cause the problems you're seeing - it does look like it could be a firewall issue though.
Edit: typos

[#3]

You say there is no Internet browsing how does this manifest its self what is the error and what browser?
If its just the browser does another browser work ie Firefox? When this happens can you run from the cmd prompt ping tracert etc and do they work with names ie ping www.plus.net as well as ip ie ping 194.119.131.66.
Can you ping from the router
The problem is the same as dns cache fault You dont say which O/S see http://www.helpwithwindows.com/WindowsXP/tune-24.html
It could be the tcip stack at fault see http://www.xp-smoker.com/freeware.html but use as a last resort
What are your dns setting on the pc try using 2 isps instead of one try 194.119.131.66 and 208.67.220.220

[#4]

I'm concerned that you might be connecting at times on the same username and password from both properties, that will cause a loss of data transfer on one of the lines for sure. If it's not that it could be related to the routing issue we had just before the weekend.
Hope that helps,

[#5]

Hi all,
Thank you for your comments, fortunately but unfortunately the problems seems to have been resolved for now at least.
The consequence of this means further testing is no longer available.
To anser the questions that were posed:
*pinging was attempted via router and cmd, resulting in failed ping to google, and dns server
*router unable to resolve dns, primary & secondary dns server down
*router recieves valid ip address from plusnet, all clients recieve valid ip address
*Software & hardware Firewalls were checked to ensure correct config
It is possible that orbrey has the answer in duplicated username & password usage......is this likely to be the case if i use my password to access the internet via dialup while my broadband router is still connected at home? if not I would lean more towards the routing issues
Thank again for all your comments & support
Kevin

[#6]

ignore

[#7]

Quote

is this likely to be the case if i use my password to access the internet via dialup while my broadband router is still connected at home?

Yep, that'll be doing it (assuming the dialup uses the same username) - if it is, as soon as you connect the dialup your workplace will lose all data transfer.
It might be worth signing up for a free dialup account to stop this from happening - it's the same pay as you go component on the free dialup accounts as it is on broadband accounts (we add dialup access in order that you can still connect if there's a fault).
Hope that helps,