Coral III Rebooted Went out of DUP mode

[ringydingy]

I have a Coral III with redundant 5vdc, 4GC, MEX IP2 running in Duplication mode, software ver. 15.85.47. A few days ago the system rebooted and came back up but had gone out of DUP mode. The system is running on the Master side, the Slave side is still up. Alarm on Master indicates "Duplication 2nd 4GC". It did the same exact thing last August 2011. Vendor suspects Slave side - "could be 5vdc CPS power supply, or 4GC, or MEX IP2, or PPS supply in cabinet". All power supplies test OK, but to start with I'm purchasing another 5vdc CPS supply. The system has redundant rectifiers and battery bank for power. I could just put it back in DUP mode one of these nights (and see what happens) OR save money and just leave it out of DUP mode (at reduced processing capacity). Any ideas what could have caused it to reboot?