I am having similar problems. I am trying to download an episode of Grey's Anatomy, but am recieving error -100008.
I have tried to download it with my iMac and my macbook both recievce this error.
I contacted iTMS support, they signed in with my account and were able to download successfully They say its a problem with either my comps, ISP, or my airport network. Both computers are connected via airport express.
I have check for Mac OS X updates, both computer are up-to-date. I have reset my airport to factory defaults, and updated its firmware. I reset my cable modem. I've done everything. Nothing helps!!!
20'' iMac 2.16 C2D, Macbook 2.0 CD Mac OS X (10.4.8)