You're referring to erratum 2/3 which deals with ARP traffic. As ARP traffic is also used in non-DNS requests I don't think this is the source of your problems. But erratum 1 is a possible source. It describes a race condition where the reception of a UDP datagram occurs almost simultaneously with the sending of a packet. The code of the Ethernet library only check for the SEND_OK bit in the interrupt register, which may never happen (at least the erratum says that). The recommendation in the errata document is NOT in the Ethernet library yet.
I'm not sure if your problem is related to that erratum, because unfortunately the erratum says nothing about the TIMEOUT interrupt. If the TIMEOUT occurs as specified in the datasheet, the code in the Ethernet library is correct.