Hi everybody!
I am new on this forum, I've noticed the thread and read the posts..
Has anybody a little bit of confidence with the command line?
..the old prompt of the beloved DOS?
There is a simple command that allows to send instant messages from a pc to another machine on a LAN.. You can try it with 2 pc:

from the prompt:
net send name_of_pc "message text" (without the quote)
[it works even with the name of the user]
Replacing the name of the reciver with a * sends the message to everybody on the LAN. (eh, the university.. what a good time!! )

Now, I never tried this with an IP... but it could work...
I've experienced the same problem for a certain period but never care..
Then ceased at all.
Maybe it can be a bug on the settings of the internal WAN of my provider (I got ADSL) that someone fixed.

so.. After visiting the site... send an e-mail to your provider explaining the problem.. and wait..