OS X :: Certain Web Sites Will Not Load On Mac After A While?

Frequent reader of Mac Rumors but I finally joined as I searched a few threads that had similar issues but didn't create a solution for me. Essentially, certain web sites will not load on my mac after a while. When I first got it and subsequently formatted and reinstalled the OS (due to the same issue I'm having now), this issue did not happen.I have a Mac Book Pro, OS X 10.5, 15.4" and other than this software screwup problem, everything runs like a dream.

Here is what happens:
I browse many websites, download files, normal computer usage, and after a while certian websites instantly don't load. A classic example is [URL], which didn't load the last time I had this issue, in addition to [URL], which also didn't load the last time I had this issue. BOTH of these pages used to load fine. The pages instantly go to a "cannot find server" error, on safari, and firefox. However, on parallels, the pages DO load on XP using firefox and internet explorer, although I could have sworn they didn't the last time...which may end up being the case as time goes on.

I have no spyware, firewall, or other network software on the machine. I DO have peer guardian but the problem persists even when I have quit the program.Restarting does nothing. Honestly, I must have done 50,000 google searches on this issue, and different people have different solutions to similar problems (permissions, etc), but this is something new, because the pages used to load and no do not.