That is odd. Are there no other log messages around the time of the reboot? Anything from before the reboot?
Since you can somewhat predict the occurrence, could you leave a system connected to the console port logging the output from there to see what happens? It may have some better info.
From the general symptoms it sounds like a power/environmental issue somehow, perhaps it's enough to trip the 5100 but not a UPS, though at least for me, my 5100s don't seem to be phased by small power blips even without a UPS.
If you leave the console connected and there is nothing logged before the reboot, then it would almost have to be power or hardware. A hardware issue would almost certainly be more random/less predictable, though.