First, nice set up! Can I borrow a screen?
It could be a number of things but lets start with your Audigy NX. If you are running digital, set it down to 48 from 96 Hz.
1. USB cable to the Audigy NX make sure it is 2.0 and that it is in fine working order, what distance are you running?
2. Make sure it is dedicated, no USB hubs if possible and unplug any 1.0 USB devices that are on the same USB line meaning any 1.0 on the card or header that is inside your carputer.
3. Make sure you have the latest drivers from Creative Labs, they did an update about 6 months ago that really affected the NX's performance for the better.
4. Turn off all VIS, background apps, etc to give max processing power.
5. Confirm that the internal USB hub is 2.0 inside the carputer and the drivers are correct.
Turn off CMSS, EAX or anything that affects the Audigy's processing as as starting point.
You could also try to use Line out to Aux RCA's instead of Digital to see if that is the problem, again if you are running digital.
It sounds to me that it is a USB issue off the start.