MS ISA 2000 asks for authentication from Clients frequently

We are using MS ISA 2000 with 2 NIC teamed.It frequently ask clients for authentication and users are unable to browse.After removing the public dns and flushding the DNS and regsitering DNS.And then adding the public DNS resolves the issues.This doesn’t have a dfined frequency.Can you please suggest a proper solution for the same.
Using windows 2003 Enterprise Edition.

Regds
VVKS