Lenova ThinkCentre (A55 Tower E2180)

Hello,

I am having 5 Lenovo coumputer in my office model no. A55 Tower \ E2180 purchase 15 Jun 2009, the problem is coming in all computers the computer automaticly gone out to network. and that time we can't excess any computer if i restart the computer it will be work, but after helf in houre same problem is coming in all computer. why is coming this problem in all computer can u help me and give me the suggestion.

Thanks
Prashant Sharma
Muscat, Oman