Network Connection Problem

Hi all,
I have recently added 50 DELL Vostro 1310 Laptops in to my network. I installed Windows XP Pro and all my programs in one laptop and used Norton Ghost to replicate in to other laptops. All laptops working fine but after few hours my Ethernet connection in the laptop shows Network Cable Unplugged then I have to shutdown the Laptop and restart to connect to the network. Any answers please…
Thanks