The above situation is happening..when..at full load..or normal Idle..?
I guess THIS is the model you are using. It has a max output of 300W (Full load) which it can sustain for around 3.3 minutes.
At load: (rough figures/not exact)
4850 ~133W
E7400 ~65W
HDD ~12.5W
Monitor ~30W, RAM (nothing to do with load/idle)
RAM ~ 10W
And of course the PSU and E-HDDs, USBs, IDE Drive will also bleed some WATT load from the UPS. You are hitting the threshold of 3.3 mins, so the UPS cannot sustain it long.
This is something we in India should be careful off. We plan ahead (which is really really good), for a good PSU. But forget that the PSU pulls direct feed from our UPS (most of us use a UPS in India --- bad power uptime). Same thing happend with me..I got a TX650 and set up a cross fire system, and then realized that my APC 650VA (max 390W) cannot support the system at full load. Had to go get a new UPS.