Thunderbird2 appears to overwrite e-mails

I downloaded TB2 beta 2 to give it a try and found that every alternate e-mail in my in-box looked as though it had another message superimposed on it. When RC1 was released I un-installed the beta version and installed the RC1 version but still got the same problem. I have installed it in several thumb drives, always with a similar result - some messages look as if there is another message superimposed on it although now it is not every alternate message but just random messages.

Has anyone else had this problem and how did they resolve it (if they did)?