I have noticed a problem in yahoo, Let suppose, you re log in yahoo messenger and suddenly due to your electricity break down you pc go off, but here is problem you will stay log in, and you will automatically log out after 15-25 minutes and your buddies in will think that you are busy and not answering, some time this thing create great problems,
hmm that's a bit odd. but it can be possible. not sure why though.. i use a different chat site and after a power failure, it takes 5minutes or so before i get signed out.. maybe you should try to contact people from yahoo technical team or something like that,,