DAC St-t-t-t-tutter?

This is probably simple and I have my theories, but would like to know how & why this happens:

New DAC (will remain unnamed, but very well respected.) Direct connect to amp.

Feeding source from a 2008 MacBook Pro via USB, 2TB RAID-1 drive connected via Firewire 400. (This source will be upgraded soon*.) Pure Music is upsampling from iTunes. On the MacBook, most extraneous functions have been knocked out, leaving the machine pretty much free to play music.

Things sound great most of the time, but every now and then, the music starts to skip, stutter, and generally scare the life out of anyone listening.

Sometimes a restart of PureMusic/iTunes will fix it, but more likely I can clear it with a simple power cycle of the DAC.

I don't know if this is only happening on HD tracks, or not.

So, what's going on when this happens? Which part of the chain should I be looking at to solve the problem? Any way of avoiding it for the time being?

Thanks!

* Upgrades will likely be a Mac Mini with SSD, Empirical Audio's Off Ramp 5 USB converter running I2S to the DAC.