Problem to connecting to server

I am using OpenJMS 0.7.6.1 on Windows.
 
My application creates connections/sessions on the fly and they are closed when not needed anymore.
 
Some of my users are getting a Connection Refused exception at times:
 
java.rmi.ConnectException: Connection refused to host: 158.228.179.171; nested exception is: 
java.net.ConnectException: Connection refused: connect
 
However, the server is up and running.
 
What could be causing the server not responding? Could it be a problem with the load of the machine on which the OpenJMS server is running?
 
Thanks!
 
Jean-Francois