Learn to use the md5 check. The faq explains it, although a quick Google search could have saved you the secod and third long downloads. If you still have those iso files you might find that you don't need to download again at all (although I expect you do, but I would styill advocate the test).

The faq also points you to the important key to getting good speed out of BitTorerent (although the BitTorrent home page also explains the importance of correctly setting up your firewall). Without this BT will crawl, with it BT will download much faaster than any mirror I've found, and will avoid the common mirror corruption problem. BT does start slow and picks up speed, so don't stop it after 30 seconds thinking "this will take all day" if you properly configured your firewalls/router.