I've just connected the EATON UPS to a different PC (250watt load) with a different PSU and tried again to
recreate the problem and captured some transitions.
First a very well behaved transition : (blackout)
...and below the real problem of the EATON UPS captured: (brownout)
Power was lost for 240ms !!!
The UPS is definitely faulty.
(Looking at the graph can you think what can create this type of behavior? )
But the initial problem of my PC rebooting had
two causes. One was the actual hardware
fault above and the second problem was with my PSU. But not because it was of low quality.
On the contrary. It is the Corsair CX500. (I have been using it for just over a year.)
http://www.corsair.com/en-us/builder-series-cx500The "problem" is, that is has active PFC for increased efficiency.
This type of power supply does not like square wave approximations
and the output from the EATON is not even close to an approximation...