I suspected this.
Anytime things are moved... GLUE records are required to do it seamlessly.
If they don;t add in the Glue Records for *ALL* the possible entries, and especially things like CNAME entries get confused.
Just do you know they only have a Glue record for ns1.abac.com not for ns2.abac.com (in fact ns2.abac.com is showing any IP address when a whois is done)
I use Aplus for my domain name stuff... but host my own DNS authoritative server, to avoid these type of things.
I also use dyndns paid secondary DNS hosting for scalability.
Good luck. DNS is picky, especially lately with all the attention its been receiving about DNS SEC and the problems being caused by people trying to poison caches to exploit IE browser phishing hacks.
If you do a "whois -H gregfolkert.net" you'll see the glue records as listed as my secondary DNS provider also publishes them directly as alternate IPs.
whois -H gregfolkert.net
[snip]
Domain servers in listed order:
ns2.mydyndns.org 209.69.32.138
ns3.mydyndns.org 66.37.218.208,64.71.191.27
ns4.mydyndns.org 212.100.224.176
ns5.mydyndns.org 66.37.218.208,66.151.188.46
uno.gregfolkert.net 208.64.37.170
