Outlook2000-Exchange connection problem

We are using Exchange Server 5.5 running on our NT 4.0 Server. Outlook 2000 takes about 1 minute to load after clicking the shortcut on a Windows NT 4.0 Workstation. It takes longer on a Windows 98 workstation and less time on a Windows 2000 workstation. In order to get it to load that fast, we had to change the RPC_Binding_Order by moving ncacn_np to first in the list on each workstation in HKEY_LOCAL_MACHINE, Software, Microsoft, Exchange, Exchange Provider. Sometimes Outlook fails with the message “Network problems are preventing connection to the Microsoft Exchange Server computer.” How can we correct this problem?