Thanks for the quick answer - not a cheap UPS unfortunately - it is an APC 3000RMT XL - double conversion pure sine wave - running from a Lithium battery bank good for approx 8 hours run timeI have seen this behavior on simulated & non sine wave UPS's... ie. low cost UPS's. The ICX 6610 psu is pretty sensitive to power. Made no difference running a single or dual PSU. Made no difference rev A or rev B or a mix of rev PSU's. I have seen the switch stay up for 1 or 2 days or up to 20+ days but eventually it gets unhappy on the low cost UPS's and randomly restarts.
Moved the 6610's to an EATON 2200 and the cold start/random reboots disappeared. They also disappeared just running them on street power. I put them back on a cheap APC or Cyberpower UPS and they periodically reboot.
So does COLD start actually mean a full switch reboot ? i.e. loss of power