Official comment
@Shalev > Sorry to hear that -- in those situations it's possible that you have another process or task running on your computer which is taking system resources away from rekordbox, thus causing buffer underrun. Please ensure that you have disabled any antivirus / malware or other system scanners (and that your computer is free of any infections too!), disable any file indexing utilities (such as Cortana), or 3rd party applications which don't need to be running. You may also try increasing the size of the audio buffer within the rekordbox preferences to see if that improves the situation.