you have a grounding issue. Something as simple as a static shock is knocking out the ADAT sync, which is causing it to freeze the D8b wont give you any indication like in Pro Tools for example when sync is lost...but, i have seen this issue in another studio i was called into. They had similar issues with the ensemble loosing sync and audio loss. Try switiching to tdif cables instead of adat cables if you havent already.
I really dont think it has anything to do with the physical connections you mentioned (midi, ehternet, spdif) but if you have POE eithernet, or some kinda grounding issue in your circuit then that would definately cause the sync issues. By disconnecting those physical connections, you only removed the problem from the circuit. plugging those connections back in may cause the loss of sync again...I would nect investigage your word clock...digital sync signal chain and settings.