OS X V10.7 Lion :: Mail "Looses" HTML Formatting Apparently Randomly

I use Mac OS X Mail and generally have very few issues. However, I ocassionally receive emails which I know are formatted as HTML, but they nevertheless show up as text... and sort of jumbled text at that (i.e., with URL links expanded inline and other issues that cause the email to be difficult to read). It doesn't happen extremely often, but it does happen often enough to be annoying. I have Mail sync'ed to several accounts: one POP3, and two IMAP as I recall. It has happened with emails retrieved from each service at one time or another. 

As I understand it, an HTML email usually also contains a text version within the content... and each mail client has to recognize a given email as either HTML or text (or RTF). Indeed, I can view the raw source for the email in Mail, and I see in the payload of the email statements such as "This is a multi-part message in MIME format" followed by sections denoted as Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: base64 and later as Content-Type: text/html; charset="utf-8" Content-Transfer-Encoding: base64. It just seems that Mail occassionally ignores the text/html section. 

Is there a way to force Mail to switch display of a given message to HTML or to text? I see options to select "Text Encoding" under the Message menu bar option, various stylistic choices under the Format header, and even the View Raw Source option under View | Message menu bar option. This last option (View | Message) also has some options regarding display "alternatives" (plaintext, best, next, etc) but all are greyed-out. Perhaps this is the issue and Mail is simply not understanding the HTML due to a slight mistake in the format of the content and it thinks it is only displayable as plain text?

Info:
Mail, Mac OS X (10.7.3)