this is beginning to sound more like a connection problem than an AIM/MSN/computer prob. does your cable signal drop alot/get massive slow downs. You might need to get the Comcast guys out there to tune it up some. I assume you're behind a router. just for curiosity's sake, do you think you can put your computer in the DMZ and see if things work better? hmm, also, for diagnoistics sake, use
http://www.ethereal.com/ to sniff the packets on your computer (set up a filter to only sniff AIM packets) and send the info to a log file. I'm not sure that I'll be able to help you with the output from this, but the servers might be sending an error message that the client isn't displaying that would be helpful for figuring things out.