Useful advice (not "switch your browser!"):
When you have issues accessing the webpage, do the following - open up a command prompt (Start->Run->"cmd") and type "tracert
www.yahoo.com" or whatever the site is. See if that completes successfully.
If it does, it means that the issue is not with your ISP. If it's not, I'm not entirely sure that the problem is. However, I've seen ISPs (including mine) have
chronic network performance issues where routers consistently go down, day after day, denying access to large netblocks for hours at a time. That could be your issue.