Hmm, interesting findings indeed. I assume you meant to write you have a DP45SG but I have the DG45ID which uses the same sound chip/codec and using the latest Intel provided audio drivers, when using optical audio output (and only that output), the audio locks up from time to time. Have you tried uninstalling the Intel/IDT drivers altogether and using just the Windows native drivers - that works for me. It'd also be worth ensuring that all of your system BIOS settings are default and that no overclocking is taking place.
So if your uninstall didn't work, reinstall the Intel/IDT Audio drivers for the board, reboot and then uninstall them again using Control Panel Programs and Features. Without the Intel provided drivers, as long as you have a reliable internet connection, after rebooting the PC after uninstalling the second time, windows will detect its audio driverless and download its own (Microsoft provided) drivers off the internet automatically.
Is the PC running with default BIOS settings (no overclocking)?
How does that work for you?