>What happens if you shut everything down for one minute?  And, what are the
clients?  Some of the newer Windows will "detect" a Internet connection, and
if it goes down, the system goes offline.

- The clients are mostly from windows XP to windows 7 and few macs. The ones
with problems are windows 7 clients (PC and laptop) and one Mac. But after a
long shutdown (1 minute) the windows worked back normal, have not tried with
the mac.

>The system runs from memory, so a corruption of the image is not likely. 
>  However, with bad power, a corruption of the memory can happen.  Also,
you could have bad memory.  Get a UBCD or an Ubuntu CD and boot to memtest
x86+ and see if your memory is good.  (Let it run for two passes at least)
You may also want to look at a line conditioner.  This is more than a cheap
UPS...  But a cheap DIY way to get one is a car battery, an inverter, and a
battery charger.  It will be noisy power, but perhaps cleaner than what you
have.

I tried different memory checker and the memory is OK. At least it is good
to know that the firewall runs on memory :) And I am getting some secondhand
cheap UPS and just replace the battery to handle this :) But reading from
your info, it seems that it may be even better to allow power failure to do
the automatic shutdown from time to time to make sure the memory refresh
frequently :) (just kidding .....)

Thank you.