If you are using the limewire player you will probably find all sorts of faults... It is not very good! If you are using a good player then there are some bad files out there and sadly many users do not check these before re-sharing ... so a lot of bad copies are out there. You may have to try a few copies to get a decent one, but in general this applies to modern popular rub... sorry songs
If you tend to go for the obscure stuff, as I do, this does not seem a big problem ... just on few tracks, and if it's very rare and there is only one "master copy" sadly the whole lot could be faulty ... Let us know if this helps.