Windows XP networking problem

My system is as follows:
Intel P4 2.8C GHz CPU w/ HyperThreading, 800FSB
Asus P4P800 w/ Bios 1007
2 x 256 MB Kingston HyperX PC3500 DDRAM
80 GB Western Digital WJD800 HDD.
TDK 40x 16x 32 CD-RW
U.S. Robotics 56k PCI Faxmodem.
Running Windows XP Pro, Service Pack 1.

I have set up a Lan Connection at my workplace using the 3Com gigabit Lan port on my motherboard. I have had the network connection arranged by our network administrator. The problem is that after about 5 minutes of internet access, I am suddenly cut off. No more access is available until I restart my computer. In addition, if I enter the Network Connections window via the Control Panel, and attempt to disable the connection, and re-enable it, the following things happen: The connection will not enable (I get a message saying connection failed) and the computer will not shutdown. I have to manually reset the computer to get things going again.

What can I do to fix this? Is my motherboard faulty, or is it a software problem?