USB - SPDIF on Eastern Electric DAC

I've recently gotten the upgrade bug and am looking to replace the stock USB input of my EE DAC with an asynchronous converter. I've tried a few USB to SPDIF converters, namely the Halide Design
Bridge, and the Stello U2 (which isn't asynch) fed into the RCA SPDIF input of the DAC. I've been having
some issues with the sound dropping out suddenly every 30 seconds or so
with both units. This doesn't happen at all when I use the stock USB
input, but does happen in both the RCA and BNC inputs when fed by the
USB->SPDIF converter. At first I thought it might be because I was using
a long USB wire, and the USB-> SPDIF converters (both being passive
devices) weren't getting enough current from the USB port. However, I put a 1 amp powered USB hub into the equation and also tried shorter USB
cables, but am still having the same problem. Have anyone ever heard of this problem or know of any remedies? Also I am using a macbook pro with OSX 10.6.4 and itunes if that helps. I'm thinking of trying either the musical fidelity V-link or ART legato, but am wary that these will have the same problem. Has anyone had any luck with any usb to spdif on this dac. Thanks, greatly appreciated.

Ron