The Mac does occasional maintenance and runs services in the background. This can interrupt music.
Try doing a PRAM and repair of disk permissions occasionally.
A weekly reboot of everything once a week is also helpful. [these are computers after all.]
Drops when playing by USB are, as Paul mentioned, due to the Mac and player programs.
Mac OSX also gives priority to finder, safari, auto updates and a few other goodies, so make sure they are turned off.
Many claim that SL sounds better than Maverics.
It used to until Mac reactivated Integer Mode. If you have intel and 4g ram, then update, if not add some ram and refer to the above which help in either case.