We've experienced similar problems with our Unity configuration and gone back and forth on whether it's a hardware or a software issue. We're at the point of doing a conversion of our secondary server to a primary, then rebuilding our former primary as a secondary.
I'm curious to know if the primary server is hanging hard when you experience the issue. With ours, the system is still able to establish a TCP connection (i.e., telnet 25 will complete a handshake) but the service running on the port never responds - just a cursor. We have to hard power the server, either via button or iLO.
Are you getting this error “Installer User Interface Mode Not Supported. The installer cannot run in this UI mode. To specify the interface mode, use the -i command-line option, followed by the UI mode identifier. The value UI mode identifiers...
The below trick might come handy when you have to add a new node to a cluster but you don't have or is unsure of the security password for the publisher. This procedure has been around for ages.
1) Login into the CLI of the Publisher.