Do you get this with the windows XP machine (if still available?) the confusing point is that this issue exists across multiple machines, multiple routers, and multiple ISP's. The issue could be the side effect of an infection on the local network which is re-infecting new machines. An interference issue, a fault caused by anther device linked to the network that is "Flooding" your router. An incorrect entry on the routers isp configuration (cable/adsl). An intermittent fault with your line. Or conflicts caused by running multiple firewalls. So to try and narrow this down a bit further, I'm going to ask a few more questions:
Do you have any other devices (iThings etc) connected?
What firewalls are running (Windows Firewall and/or 3rd Party Firewall)?
Have you tried using goolge chrome?
The issue only exists intermittently while opening new tabs in IE9.
Have you tried using IE (without Add On's) option?