I, too, am experiencing this problem and have been experiencing it since I shifted to Lion in October 2011. Among many other issues with MacMail, including messages jumping back into mailboxes after I move them and the unreliable font problem and the random signature issues. I don't have constant internet access, and having a program to read and respond to email offline is crucial. I also don't understand why Apple, with such a well-designed OS, has such problems designing a simple Mail program.
Submit bug reports, people.
FYI, I have the problem even copying and pasting even fragments of text within a paragraph of an email, and it seems totally random.
My two cents, though--if I convert to plain text, I can move the text block to where I want and it doesn't jump. Once I have the raw text in the correct place, I can convert back to rich text. I don't need rich text for my purposes, so I tend to just leave my emails plain text. What all of it does to my signature, I have no idea, and I'm not sure I want to know. They're probably being changed to 24-point bright red Comic Sans in my professional emails.