Is this the real AIM application you are using or iChat with the AIM service?
AIM (The Application) on OSX is VERY buggy and often crashes. I refuse to use it on either OSX or Windows for that matter. If AIM is all you are after why not run iChat? Works great with AIM and is very stable. I keep it running 24 hours a day here and it's never let me down. If you need a Multi Chat client try Adium. Very good and stable Multi Chat client that supports AIM, Yahoo, ICQ, MSN and more.