Just an update:

The problem was the DSP CD20 board. I now have a brand new card (under warranty) installed and have tested it for over a month with no issues.

Theatre Room: Sony 4K VW500ES, 861v8+ID41, HD621, DSP6000's, DSP5000C, DSP5000's, Oppo 205, Sat STB, XBox One X, Stewart Cinemascope Screen, QNAP TVS 1271U-RP (96TB) with Sooloos/Roon core pkg.