hrmm I helped a guy with this last time..
here goes..
you kind of added a boot bot to your list either when you log into a chat room the bot triggers and boots you off or when you log into yahoo it triggers and locks your account if you do not have a bot on your list you either have pissed off someone and they are seeking revenge on you by booting you off Yahoo and locking your account. there is another reason why this might be happening you said that your account was locked up for 2 weeks in that time the so called hacker used your account and probibly talked trash and got booted from the room constantly.. hence the word contantly.. if you get booted out of a room more than 5- 10 times you are banned for life unless you contact yahoo. so either check your list for a booting bot or… send yahoo a email.. but the best remedy is to save your contact list.. and make a tottally new account ..
hope that was enough info.. 🙂