A bit of a strange one....

[#1]

..... (which is a phrase used about me too!)
I'm a relatively new PN user, having suffered with Sky for 10 months after they took over O2, however that's all in the past now.......
I run a relatively (for a home) large LAN with a variety of devices connecting to it running differing O/S:  Windows 7&8, Ubuntu, iOS and Synology DSM 5.0.
Pretty much all but the iOS devices are connected via gigabit ethernet, although some of the Win 7 laptops occasionally connect via wi-fi.  All relevant O/S updates have been applied to all machines and all the windows machines run the current version of Avast! (2014.9.0.2021).  Firewalls are turned on with default settings (Windows own flavour on the respective machines and ufw on the Ubuntu machine)
Here's the problem.  From time to time (there is no set pattern), the WIn7 machines will fail to connect to the internet - chrome gives the standard "connecting" message in the bottom left corner of the window.  However I can connect to any device on the LAN without problems.  When the Win7 machines are internet-less, the Win8 one is fine, as is the Ubuntu machine and the Synology box.  
The Win7 "issue" can last anything from 10s to 30 minutes and will go as suddenly as it appears.  During this time disabling Avast! has no effect, neither does flushing and renewing the DNS cache.  Wireshark does not show anything untoward in the datagrams between the Win7 devices and the Router (a TP-Link W8970) and replacing the router with the PN Technicolour one makes no difference.  Although I have 20+ years experience in IT and telecomms, I was never a datagram guru so I might have missed something - any Wireshark experts out there that can suggest some monitoring filters to apply / or what to look for?
The network connection logo in the taskbar will also show the orange exclamation mark and report "no internet access" during the outages.
Any suggestions?
Thank you, in advance, for your help.

[#2]

Quote from: djf

... I run a relatively (for a home) large LAN with a variety of devices connecting to it running differing O/S:  Windows (7&8), Ubuntu, iOS and Synology DSM 5.0. ...

You are me, and I claim my £5 
Only thing I can think of is loss of DHCP settings (gateway, or dns servers). (Are you using DHCP?). Is it worth trying manual TCPIP confi rather than DHCP for the win 7 machines - (Even if just a test). You'd need to reserve the IP addresses in the dhcp server though.

[#3]

I used reserved addresses, configured at the router, ie MAC specific (that way the servers etc always get the same IP address) and all the clients are set to "obtain an IP address automatically".  DNS servers are the PN specific ones (212.159.13.49 and 212.159.6.9).  As a test I set one of the Win7 machines to a fixed IP address and changed the DNS servers to Google's (8.8.8.8 and 8.8.4.4) but it made no difference  
I have also deleted any DNS server entries from the router (so it should pick up PN's default entries) to no avail and have even rolled back (via system restore) to a snapshot taken on Monday - again to no avail, which rules out a MS update issue.  Up until then, everything was pointing towards it being a Win7 issue, but now I am not so sure.  All I do know is that without a working Win7 laptop then I'm bugg3r3d for being able to work 

[#4]

I'm clutching at straws here but there was a problem the other day with IPV6 being enabled on the TG582. Now Win 7 will have IPV6 enabled so I wonder if it's worth disabling IPV6 on the TG582 just to see if it has any effect. See this thread http://community.plus.net/forum/index.php/topic,132001.0.html.
NB the TD8970 is alos IPV6 capable so that MAY explain why its no different with that.

[#5]

Thank you, everyone, for your helpful comments.  All is now OK.
Bypassing the router (PPPoE direct into the BTO modem) was a success so I isolated every component and started again.  Turning off allocated IP addressing in the router made no difference but removing the other router (the Technicolour TG582 supplied by PN, which had DHCP turned off and was being used as a wireless AP) caused the problem to go away.
So... back to using my old O2 Wireless Box IV as an AP (DHCP turned off) and all is well.
It seems that even though I configured the T582 as a dumb AP (DHCP off, IP address fixed to fall within the range allocated by the W9780) it rebelled!
Thanks again for your help.

[#6]

Quote

It seems that even though I configured the T582 as a dumb AP (DHCP off, IP address fixed to fall within the range allocated by the W9780) it rebelled!

Strange!, I use one in exactly that configuration and it works fine. One difference would be that I don't have any Windows 7 machines with IPV6.