When you say it keeps happening, are you saying that after it fails, you remove the corrupted partial download, the download starts again from 0%, and it progresses to another point before it fails? Many people are deleting the file from iTunes and assuming that it removes the corrupted file. It doesn't. If the file keeps failing at the same point, you may not be deleting the file. See instructions above in this thread for removing it and restarting.
If you are deleting the file correctly, then the root cause of this is an interrupted download. Are you downloading the files wirelessly? Or is your internet connection a bit flaky? Test your connection with a broadband speedtest site (like
http://www.speedtest.net/) and post your results here. If everything looks okay, then I'd scan your computer for viruses / spyware to make sure you aren't losing CPU cycles that could be interrupting your connection (e.g. being part of a botnet). Good luck!