I see, since you are constantly working on boxes, they are assigned IP leases.... after so many computers, the DHCP server runs out of IP addresses to assign, not letting you connect to your network.
That is an easy fix...
What you can do in that case is release the IP address manually from the computers after you are done working on them.
Most of the time IP leases have an expiry date it could be a day or a week (depends on how the DHCP server is setup) but it if you work on enough computers to fill up the DHCP Scope you're denied access to the network, which sounds like your problem.
In order to release the IP address in a Windows/DOS environment, you go to your command prompt and type in "IPCONFIG /RELEASE"
That will tell the DHCP server that it can reassign the IP address that was being used on that computer you were working on.
Also, if you find that this is happening quite often, speak to your systems administrator and ask him to make the DHCP Scope a little bigger for you and to reduce the expiry time for the IP leases.. that should sholve your problem.
Let us know how it goes.