Uno R3 connected via Ethernet shield becomes unreachable after a few minutes
I don't know where my compiler was finding the UDP code in the above sketch, because it's not in EthernetV2_0.h. The problem was fixed by adding the following lines, from the example sketch UDPSendReceiveString written by Michael Margolis and included with the Ethernet_Shield_W5200 library:

#include &lt;EthernetUdpV2_0.h&gt;

#define SDCARD_CS 4

void setup():
    //...//
    pinMode(SDCARD_CS, OUTPUT);
    digitalWrite(SDCARD_CS, HIGH);