Sorry for the late reply. We've stopped monitoring those ports with Nagios, and the problem disappeared. I will try to set a test environment in a virtual machine to try and reproduce the problem, but I won't have it at least before a week, since we are quite busy at the moment. I will also try to reproduce the problem in a JBoss installed in my development machine. I tryed doing telnet to a local JBoss and the problem is the same, but I need another one running in a VM to see if they can communicate.
Let's see if we can nail this down, 'cos now I'm really curious to find out what is happening!
I'll write back when I have more info on the matter.