I think I've tracked down the cause of the Â appearing in posts apparently at random after they've been saved or restored. I couldn't find a place to make a bug report, so I hope this does okay.
The problem seems to be related to posts that are pasted into Word Press, possibly only from Word, where the sentences have two spaces after the punctuation.
A simple solution would be to have the conversion utility automatically replace all double spaces (whether typos or deliberate) with a single space. Since no browser displays two spaces appropriately anyway, there would be nothing lost.
Hope this helps.