Do any of the tracks not start at time zero (there is empty space before the song starts)?
If so, try pressing HOME on the keyboard then drag select a little way from time zero and Generate > Silence.
You could also try Audacity > Preferences: Recording and try different “Audio to buffer” settings (they affect playback too).
If all your tracks start at zero, did you install Mavericks over earlier OS X? Reinstalling Mavericks may help cure audio problems.
On Mavericks, Scarlett recommend using standard Core Audio USB drivers. See: https://forum.audacityteam.org/t/recordings-on-playback-choppy-skipping/32411/15 .
Gale