I think it's tied to multipart vista emails.
I've got some new body decode code I'm working on that I'm hoping to solve that issue.
The decode file that I'm using now is an old pear package that has reports of bugs along those lines. Noone that I know of has been able to get the code to work so I'm again, building from scratch and I'm going to try more of imaps functions with some simple base php decode functions.
When you notice it happen, please verify that it is a multipart email in the header information of the email