I carried on reading on the MSN protocol and now I understand the PNG commands are actually being sent in order to keep the socket alive even if idle.
So basically what's happening to me is that the commands are repeatedly being sent but do not get acknowledged by the server, causing the socket to close down and in order shuts down the connection and logs me off.
Reference:
http://www.ezonate.co.uk/messenger/MSNP15:Pings
Any advice on that? :|