I have problem in my server?

i have problem in my server have installed windows 2003 server, after start the server i can ping my users from my server...but after 1 or 3 hours i can't ping my users...this NIC card goes down...again i unplug the network cable...put again it will work for 1 or 3 hours...then again this problem come...i change the NIC card also...it problem gives again...plz help me on this...wht to do.