Cannot add new printer port

My NT 4.0 server is used as a file and print server and I use JetAdmin 3.4. I have installed SP6a, but now I am unable to add a new printer port. Every time I click the new port button nothing happens. Every once and a while the desktop locks up and the spooler shuts down and sometimes the rpc fails and I have to restart the server. How can I get this printer port installed?