I feel your pain, I have the same problem, and many others do. Nobody seems to know a solution or why it is happening.
I have this problem too. For about 5 seconds every time I send a message the messag window freezes before it goes through. I even tried trillian and I have the same problem. Nobody anywhere seems to have a solution, and don’t try reporting it to the aim people at aim.com, they don’t respond.