I would test your DAC with another software and PC than iTunes and your mac mini. There are many programs that play music....surely you can find something and just test to see if you get drop outs...in which case it would lead one to suspect the DAC.
Is there anything else that might be hogging the USB bus...do you have a dongle and wireless mouse or something plugged into USB (printer/scanner)that might cause latency in the USB port?
Is there anything else that might be hogging the USB bus...do you have a dongle and wireless mouse or something plugged into USB (printer/scanner)that might cause latency in the USB port?