Get a VPN.
If it doesn't work, try a different one
[You must be logged in to view images. Log in or Register.] Lots of people have fixed similar issues just by using a VPN (including myself, although I'm on Linux so I'm not a normal case).
Now, if it works, you can decide whether you want to keep using it (they're only like $5-$10/month, and they protect you if you download bit torrents), or whether you want to yell at your ISP (now that you have "proof" that they suck at routing) or switch to a different one.
But at least if you try a VPN (most even have one month free trials) you can confirm that the issue is a network-related one.