In Mail.app body of e-mails is blank in preview pane
Using Mail in OS X 10.8.4 on an iMac. Everything working fine earlier but now Mail app is acting up, a little slow on response and the body of all emails is blank in the preview pane to the right. If I double click on the email itself it opens up fine. I have restarted several times and also shut down completely attempting to correct. Several times after restart the mail app seemed to be back to normal but as soon as I start clicking through messges I again have the problem. I've read through some posts that were several years old that talked about deleting the envelope index @ ~/library/mail/envelope index but I do not see this anywhere in Finder.
Please help, thanks..
iMac, OS X Mountain Lion (10.8.4)