IPv6 access to cisco.com issues?

[AndrewButterworth]

I have an interesting issue at home with my IPv6 access.  I have had a tunnel to HE for over 2-years now; all working fine.  I had some initial MTU issues that I have sorted out but on the whole its been working well.  When I originally signed up the tunnel server in London was full (I am in the UK) so I opted for the geographically nearest which was France.  I have a routed /64 & a /48 and I have 'subnetted' the /48 down into various /64's that I have over various VLANs at home and I as say it all seems to work perfectly.  I am using a Cisco 887VA router on a BT Infinity VDSL2 circuit which trains at about 50Mbps down and 5Mbps up.  The only issue I have had is google.com and youtube.com redirecting me to the google.fr and youtube.fr due to my source IPv6 address.
Nothing has changed on my side for ages but about two or three weeks ago access to cisco.com began to be a bit erratic - pages would half load and stop, clicking on links would time out etc.  By default (using Windows GPO) I use a dual-stack IPv4/IPv6 Squid proxy server to access Web sites, however I also have an IPv4-only proxy so forcing my client to use this solves the issue by forcing an IPv4 connection to cisco.com instead of the default IPv6 connection - I manually change the proxy in the Browser to the IPv4-only proxy but GPO refreshes at regular intervals so this reverts every 90-minutes (default GPO refresh).  Connecting to other sites over IPv6 is fine - google.com, youtube.com, facebook.com, linkedin.com etc plus anything I search for with 'ipv6 only sites'.
I access cisco.com a lot as I use it daily in my job so this is beginning to get a pain.  What's odd is it seems to be worse in the morning and then recover in the afternoon without any intervention.

Any ideas?

Andy