Could not solve it!...
I'm having the same problem.
I've already changed the encoding (and tried them all: cp1252, latin1, iso-8859-1 and utf-8) in Calibre preferences but it did not work.
The strange thing is that the ebook looks perfect in Calibre viewer (after converting it to mobi) but in my Kindle2 it still shows the question marks.
Could you please help?
Thanks in advance!