Perform either NSLookup or Dig tests from a location getting unexpected answers:
NSLookup (Windows installations have this)
nslookup sub.domain.com.
nslookup sub.domain.com. rdns1.ultradns.net.
nslookup whoami.ultradns.net.
nslookup whoami.ultradns.net. pdns1.ultradns.net.
nslookup whoareyou.ultradns.net. rdns1.ultradns.net.
nslookup whoareyou.ultradns.net. rdns2.ultradns.net.
Dig (Available in Unix variants or on Windows if it has been installed)
dig sub.domain.com
dig sub.domain.com @rdns1.ultradns.net
dig whoami.ultradns.net
dig whoami.ultradns.net @pdns1.ultradns.net
dig whoareyou.ultradns.net @rdns1.ultradns.net
dig whoareyou.ultradns.net @rdns2.ultradns.net
AND
Perform traceroutes from a location getting unexpected answers:
Windows
tracert rdns1.ultradns.net
tracert rdns2.ultradns.net
Unix Variants
traceroute rdns1.ultradns.net
traceroute rdns2.ultradns.net
Additional Information
sub.domain.com is a placeholder so replace it with whatever host name is having issues.
These tests should be run from the location that is experiencing issues. If you do not have issues on an online testing site like www.digwebinterface.com or www.kloth.net, then the tests should not be run from there.