iMessage on Mountain Lion disconnecting AIM and GTalk
I have been having this pesky problem as well.  Mine was solved by reseting the password with the provider.

I found a post that stated that someone solved this by logging into AIM.com website and reseting their password. I tried this and found that since my AIM account was set up through another provider (Apple) that I needed to go to Apple and change my password.  I did this and it resolved my issue.  (I needed to change the Apple password anyway for good security in that I had not changed it in YEARS).