Assuming you are in a region where you are allowed to redownload your past music purchases, delete the broken tracks from your iTunes library (but do not hide from iCloud), close and 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 click the small download activity widget beside the search box and untick Allow Simultaneous Downloads. This may improve reliability in future
If the problem persists, or that facility is not available in your region, contact the iTunes Store support staff through the Report a problem service, similar links in your account history, or via Contact Support.
See also:
HT5085: iTunes in the Cloud and iTunes Match Availability
HT2519: Downloading past purchases from the App Store, iBookstore, and iTunes Store
tt2