Bogart I am not doubting your network, I am assuming you replaced the network cable on the faulty machine, and changed ports on your switch already. Also, set up a ping watcher for the machine and watch the results for when it falls off.. this will tell a lot.
There is also a Windows tool you can use, to listen to the multicast packets that the Prospero(s) send out. It's used for communicating with Prosperos in a datacenter, but handy for diagnostics.
http://ecointalk.net/topic/1182-multicast-address-grouping-your-prosperos/