users saying that they are logged in as other people

hello,

i run a car blog here in brazil that has over 30.000 unique visitors per day, Noticias Automotivas. as the blog grew up, i started having problems with people not being polite to each other on comments. so i started requesting people to register, in order to be able to post comments.

did this some 6 months ago. everything went fine in this period. but some days ago, readers started to complain that they get to be logged in as other people, randomly.

each time they log in, it says they are some other guy. and then it goes back to normal. but every once in a while, the problem happens with some other user.

have anyone ever seen this happen? could this be some specific problem of WP 2.3.2?

thanks!