What could be causing microphones to randomly quit for up to 30 seconds during a meeting?

Have 2 Tesira Forte AVB VT with an EX Mod with 2 Mic/Line cards: System is 7 years old and has been running fine. In the last few months - having issue with some of the microphones randomly quit for a few seconds and then come back on. I have dumbed down the DSF file - removed all gating / auto mixers and bypassed the same within AEC. Three of the microphones are analog - hard wired to the Tesira Forte' and the other is over AVB. If they use limited mics and run their meeting less than 30 minutes no issue. With longer meeting the issue pop's up? Biamp support has does not have an answer or suggestions. There have been no other changes to the DSP. The video and control sides have been updated, but the Tesira Forte' side has not been touched. The DSP is on 24/7 - looked for overheating or fans going out, but nothing noted.