Wrong name displaying when writing a comment

My readers have experienced something strange very recently, when they write a comment, the wrong name appears once they’ve submitted their comment. They say it’s the comment of the previous commenter.

This seems to happen sometimes, not always. Registered users who are logged in don’t seem to have this problem. I think it maybe had something to do with the new WP-HASH CHECK anti-spam plug-in I installed, but I have deactivated it and the problem continues.

Any ideas what the problem is? I’m using v1.5 Maybe if I have everyone clear their cookies that would help?

Thanks in advance!! ??