I suggest you browse this forum. This issue has been going on for over a week now and there's hundreds of similar questions already. It happens to some people part of the time, some people for all the time, not at all to others. Some genres for some, others for others, etc. Different versions of iTunes and systems. There's some things that seemed to help some people, though I suspect it was just coincidental or it solved a similar but unrelated issue.
It's not clear what the issue is, or where it lies. If it's something to do with Apple's servers they are not likely to discuss it. There was an issue like this a few months ago with the weather widgets and people kept on posting with no news until one day it suddenly started working, again with no news. If you wish to, you can report to Apple.
There are other applications that let you listed to online radio if you are desperate. Check www.pure-mac.com