Eric,
It may be worth asking your service provider to look into it, the internet works in a very strange way, and it is possible that your ISP is having a problem with some sites.
Honest!
We were having quite bad performance issues a couple of weeks ago, but I tweaked quite a lot, and things look like they are running fine now.
Let me know if you speak to your ISP, what they say.
Cheers
Dan