early 2008 MacPro/UPS shutting down too quickly?

apologies in advance, as I'd bet this has been covered before - just couldn't find it with Search?

the problem is - in broad, general terms - I have an early 2008 8-core hooked up to an APC UPS XS 900 battery backup. The Mac Pro's a couple of years old, as is the APC.

Lately, when power goes down, the UPS battery kicks in just fine, giving me a few moments to save work, as hoped for. But when the power comes back on - and the UPS switches back from battery to line - the MacPro dies. The problem is that, if it's a really quick "power-off/power-on" (say, about 1 sec. or less), having the MacPro shutting down that quickly leaves no opportunity to save anything!

I would pretty much expect the UPS to be at fault here, but recent reading has lead me to believe that the power supply of the MacPro may also be part of the equation.

So just wondering if anyone else has encountered this type of problem, and suggestions?