Assuming you are in a region where you are allowed to redownload your past music purchases, delete the corrupt tracks from the music section of your iTunes library, close & then reopen iTunes, go to the iTunes Store home page, click the Purchased link from the Quick Links section in the right-hand column, then select Music and Not on this computer. You should find download links for your tracks there.
While downloading select Downloads in the left-hand column and make sure Allow Simultaneous Downloads is unchecked. This seems to help with partial downloads...
If the problem persists, or that facility is not yet available in your region, contact the iTunes Store support staff through the "Report a problem" links in your account history, or via Contact Support.