UPS electrical problem

[Greg Rundlett (freephile)]

I have my Linux "Media Center", a Dell Studio Hybrid PC, 2 1TB Fantom
external drives and a 25" lcd monitor plugged into an APC Backups Pro 500
UPS on the battery+surge protection side.

Within the past week, when the household thermostat kicks on or off the
central A/C system, the PC shuts off instantaneously.  Other electronics do
not seem to be affected, although they probably are.  It's just harder to
confirm since a flicker in the drive or monitor power would be hard to
observe.   I assume that a sag or surge gets through the UPS and affects my
PC, but don't understand how that can happen when I thought that's what the
UPS was supposed to prevent.  And, short of replacing the UPS, I'm not sure
what to do about it.  Is there something I can do to prevent this sag/surge
event -- since it's likely to affect something else?

Thanks,

Greg Rundlett
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