[Udpcast] Rogue packets?

We have used udpcast for a while at our school and it has performed flawlessly. Recently, we have cloned succesfully a bunch of Dell Optiplex 745 machines. Now, at another site, cloning is erratic. Most receiving machines get "rogue packet" messages, and on these machines the cloning seems to "hang". However, some machines finish ok. Is this a network switch problem at the site or something else? Kari Hyvönen Kerava, Finland