Could you do a little test for me? If you run the sketch UDPSendReceiveString (an example from the ethernet lib) then send it artnet, what do you see in the serial monitor
My results described here http://arduino.cc/forum/index.php/topic,96830.0.html
As you can see I get the problems even just using this udp example sketch.
Note that I’m now testing in 1.0 but have tried 21,22 and 23 also.
Flushing the buffer (with EthernetUdp.flush()) didn’t seem to work, and only made the arduino freeze more frequently.
You can get the wiz811 from rs components http://uk.rs-online.com/web/p/embedded-peripherals/6989164/ or I can send you one of mine for testing