Have to say we see no issues here with the worldnic.com nameservers, other than they appear to be located on the same physical network. I think people should post queries that fail, including date/time, and full "dig" output for that query from the server they used, and the version of recursive nameserver used. Otherwise it is purely speculative guess work to figure out if it is a DNS delegation issue, or something else (network congestion?). No one should be surprised that a DNS request may be truncated and switched to TCP, that is in the RFCs. Although the servers in question run BIND9 so presumably support EDNS0, which suggests those seeing truncation may be running rather old code, or unusual recursive resolvers. The worldnic.com and worldnic.net appear to use the YYYYMMDDVV convention for SOA serial numbers, and so it would appear nothing has changed their end in terms of zone data for at least five months in terms of zone file settings. All looks rosy from here.