Sounds like a latency issue with the USB interface. Adding more memory to the computer can help sometimes, but it may just be the driver and interface on the DAC.
I recommend doing these tweaks to any MAC:
Get iTunes iBooks:
The Optimization Guide for Apple OS X - Mountain Lion
or
The Optimization Guide for Apple OS X - Snow Leopard
Steve N.
Empirical Audio
I recommend doing these tweaks to any MAC:
Get iTunes iBooks:
The Optimization Guide for Apple OS X - Mountain Lion
or
The Optimization Guide for Apple OS X - Snow Leopard
Steve N.
Empirical Audio