VNC will not connect with DNS name

I was wondering if anyone has a fix for this.
I am running Ultra VNC on a clients computer and I can connect using his DNS name but then when I go back in after using it the same day, it will sometimes not work. I can use his ip and connect but with the dns name it fails to connect. I am using Dyndns.org for my name server. If I try the next day, everything is fine again.