I’ve noticed a problem with Messages for the last year or two: if your network drops, you stay Disconnected until you manually tell Messages to log back in. That’s pretty lame, my Comcast cable drops several times a day so I need something a little more robust than that. I found a workaround: use cron to do the work for you. Fire up a Terminal, run crontab -e and put this in it:
*/5 * * * * osascript -e 'tell application "System Events" to if (processes whose name is "Messages") exists then tell application "Messages" to log in'
This uses AppleScript to tell Messages to log in every 5 minutes. Now if the network drops, you’ll only be disconnected a few minutes.