If your country's iTunes Store allows you to redownload purchased tracks, I'd delete your current copies of the dodgy tracks and try redownloading fresh copies. For instructions, see the following document:
Otherwise, I'd report the problem to the iTunes Store.
Log in to the Store. Click on "Account" in your Quick Links. When you're in your Account information screen, go down to Purchase History and click "See all".
Find the items that are not playing properly. If you can't see "Report a Problem" next to the items, click the "Report a problem" button. Now click the "Report a Problem" links next to the items.