My internet dies after a minute or two

[splashpot]

I just refomatted my harddrive. Now, for some reason, when I reboot my computer, my internet will work for a minute or two, then it completely dies. I have no idea what could cause this. If I reboot again, same thing happens. I am on a school network.

[splashpot]

And if I unplug the ethernet cable, then plug it back in, same thing happens. I regain internet for maybe one minute, then it dies.