It could be many things, but that does sound like it could be an anti-virus problem. Anti-virus applications, by design, block access to files until they've been scanned. If the system is periodically unable to "fill" the pre-read cache quickly enough because the AV software is interfering, then it could cause random drop-outs. Any chance you'd be willing to try completely uninstalling NAV and testing? That would help rule that out as a potential cause. Assuming you have your license key and installer for NAV saved, you can always re-install it later.
Norton AV is fairly notorious for being "heavy" on the system. If the version you have is a "full protector" that includes network protection, then this could also be causing the issues if the files are stored on a network share.
It could also be a problem with the DAC and/or drivers, of course, or a network problem if the files are stored on a network share. The other thing I'd try is completely uninstalling the DAC and then starting over from scratch.
0. Make sure you have the latest Firmware for your DAC. This could certainly fix the issue if there was a bug discovered.
1. Remove DAC drivers (while DAC is still connected).
2. Unplug the DAC
3. Reboot
4. Let Windows finish booting up, so that it "clears" the drivers.
5. Reboot again.
6. Once windows comes back up, reinstall the DAC drivers and device.
Often with USB devices, you need to install the drivers FIRST, before you plug the device in, and then attach the device. However, some USB devices actually have you attach the device during the install process. Either way, don't attach the DAC until either the driver install wizard tells you to, or until after the driver install completes.
If the device is still attached while you try to upgrade the drivers, it can often "look" like it worked, but part-of (or all) of the drivers could still be stuck on the old version. This is pretty common with USB devices.