Win2k clients rebooting continually

I have a lab of WIN2000 clients. Recently they started to reboot continually and would not start. They would get as far as “Applying Computer Settings” and then reboot. If I would disconnect them from the Network, they would come up okay, but only stay up about 2 hours once connected back to the network. The only way they would stay up, is to boot them in “safe mode with Networking” and then restart them. They would then be good for the remainder of the day, until the next morning when they would reboot again, regardless if they were left on all night or shutdown. What could be causing this problem??