[Serusers] Ser question

I am having a rather odd problem. I had the Ser server working for a while, but all the sudden, strange stuff started happening. I have it running on Redhat AS3 Server, with 2 network cards, one with a public, and another with a private address. I have the sip phones configured to have the public address as the as their sip server. The strange thing is, though, when I sniff, I see the darn server get the Register request on the public interface, but sending the OK response on the private one! I tried turning the private IP interface off alltogether, but then it simply sends nothing back at all...it has been very frustrating, as it was working fine before.