Erratic Outlook Behavior - Receiving Blank Emails

I have a user on a Vista SP1 notebook with Microsoft Office Pro Plus 2007 installed. Outlook 2007 is not displaying the body of all emails all the time. For example, some HTML emails received by the user will appear blank, while others will not. Some plain text emails will appear blank, while others will not. However, accessing email via Mailgate Exchange *always* displays the messages properly.

I tried completely disabling our antivirus/security software as other posts mention (https://www.experts-exchange.com/questions/22805512/Why-would-Outlook-2007-send-out-a-message-that-is-blank.html), FortiClient 3.0.541. 541 is the latest FortiClient build and all Windows Updates are installed. No luck with that.

I found the following post, but not how to change in which format emails are viewed (can't you only change the format (HTML/Plain Text/Rich Text) of sent email?): "I have seen this where someone sends in email with only text and the receiver is viewing only in html so the text is hidden and the email looks blank.  That would not [sic?] explain why you no longer see the body." (https://www.experts-exchange.com/questions/22675504/Outlook-2007-body-of-email-disappears-except-for-attachment.html)

When this happened once before, about 2-3 weeks ago, I rebuilt the local Outlook profile and the emails all appeared fine once they downloaded again--even those that were previously opening completely blank.

What steps should I take from this point? I tried disabling the COM Add-in for FortiClient antivirus as well, but the same emails remain blank.

Uninstalling and reinstalling both Outlook and FortiClient perhaps?