This could be a number of things … Bad NIC, bad NIC configuration, bad switch or hub or even a bad cable (all of which can cause switch/hub lock or broadcast storms). There is also the possibility of a worm or slammer being embedded somewhere but you will need to narrow things down first.
1. Process of Elimination: Turn on the machines “1″ at a time, let it boot completely and see if it the problem exisits then shutdown the machine. Repeat this for the other machines one machine at a time. If it is only one machine then you can look at the hardware situation.
2. Run either NETMON or a public domain packet sniffer such as Ethereal to analyze the traffic and see what exactly is being sent. You can see if it is broadcast traffic as opposed to a flood.
3. You want to reduce or eliminate BROADCAST traffic. Make sure that you are running an internal DNS server and that all workstations and servers are pointed to it. This will reduce broadcast traffic for things such as WINS and NETBiOS calls.
Post your findings and we can look at where you stand.