New iTunes makes songs glitchy

If you've recently installed the 64-bit version of iTunes 12.1, there is an alternative downloader for 64-bit editions of Windows here:

iTunes 12.1 for Windows (64-bit — for older video cards)

that seems to address the playback issues that some users have been reporting with the full 64-bit version (the alternative installer here is the same architecture as most previous "64-bit" versions of iTunes, i.e., a 32-bit application with a 64-bit installer). The "for older video cards" label appears to be something of a misnomer as some people have reported that this corrects problems found on current / high-spec systems.

Some users have reported that after installing this alternative version it is necessary to (re)select the Windows Audio Session option in Edit > Preferences > Playback ("Play audio using:" dropdown) to restore correct playback.

Uninstalling / reinstalling shouldn't affect your library, but it is always a good idea to make sure you have an up-to-date backup of it before you start. See turingtest2's notes on Backup your iTunes for Windows library with SyncToy for a suitable/recommended method.

Note also that the latest version of iTunes (12.1.1.4) is stated to address the playback issues that many users found with 12.1.0.71 (and most users seem to have found that this is the case). I'd suggest doing the following:

• Backup your library
• Update to 12.1.1.4, using the standard 64-bit version from Apple's download page
• If this doesn't solve the problem:
  • check that you have the latest version of QuickTime installed (7.7.6) - this particularly seems to affect playback issues on Bluetooth devices
  • try changing the setting of Play Audio Using in Edit > Preferences > Playback - you'll need to restart iTunes for any changes to take effect. Most people have found that selecting "Direct Sound" eliminates remaining playback issues.
• If you're still having playback issues, there's an alternative version available here: iTunes 12.1.1 for Windows (64-bit — for older video cards) - as with 12.1.0.71, this is a 32-bit application with a 64-bit installer, and may resolve remaining incompatibilities between the "full" 64-bit version of iTunes and some Windows systems.

Finally, if you have any issues with re-installing iTunes, tt2's notes on Troubleshooting issues with iTunes for Windows updates describe the steps needed to completely remove all components of a previous iTunes installation.