That preamp is "bus powered" right? The spec for USB says 500ma max, so the box can't expect more than that. A 7805 should handle twice that. If you don't have a 10uF capacitor across the output to act as a "shock absorber" then do that, or use whatever circuit is recommended on the package you got. If you don't have the VR well heat-sinked, find a chunk of aluminium such as the CPU heat sink from an old PC, and smear some of that heat sink goop on there (comes in a small tube like toothpaste).
Any 5V/500ma bus powered box is at a big disadvantage compared to something like a UA5, V3, mini-me which has 9V and 1 amp. SD's website says it can supply 10ma of power to each mic. Although most mics only use something like 3ma, Earthworks will use 10, and I'm not sure about TOA. 20ma of 48V phantom power uses a big chunk of the power budget for the entire box, which doesn't leave much for the rest of the box. It's not unthinkable that if you have mics drawing a lot of juice, when the analog circuitry hits clipping that it could starve the SPDIF output connection voltage for a couple of hardbeats.
What I would suggest:
- put heat sink, goop, and caps on the VR if it's not already.
- put a voltmeter on the VR while it's running. If you can see it dipping below 4.75V then your power supply is "out of spec".
http://en.wikipedia.org/wiki/Universal_Serial_Bus#Power- Try it with a Denecke AD-20 power supply and see if that makes the problem go away. Not forever necessarily, but for a test. I can loan you one.
- Try other mics.
If the voltage is dipping below 4.75, then I would focus on improving your power supply. If it doesn't dip, then your preamp isn't quite what it says it is, or your mics draw an unusual amount of phantom power. But the idea of using a 7805 VR is fundamentally sound.