hi jitendera sinha is here.i am not a exprenced person but i am trying to expalin it base open my therortical concepts.
ok
the answer is depend open your office architecture.i mean to say that what type of device you uses in your office network that is switchs or router
as you say says that u have two diffrent network in you office  then i suppose that you have implement a router between two network and each network is connected through switch
ok 
now the naswer may be
1 scince there are switches in the network then there is gretaer chance to become loop.so you have to inastal stp on you fast ehernet switches
this is the primary cause that your network is geting up and down.
soln-if you donot know how to eanable stp on swirches so make switches off and after that on it then see that problem  is still pressiting it will ocuur i know but after some time not after 10 or 5 sec

2 nd sl^n -if you are using router more then one then u have to check the default timer that is hold down time of that interface by implemanting comand
router#sh cdp
after seening that u can configure holddown time for that router

but as far as best of my knowledge 1 soln is appropiate 


power opption is not the wright answer.

i am waiting for another answer plz write this is a very good problem