I frequently use eRightSoft SUPER when I want to convert something. It's free and can convert between a load of different file formats. One of the options under the avi filetype output codec is M-JPEG, which I presume is the same thing? If so it could be worth a try. The site is slightly weird, i dunno why but half the time the download links dont appear for me so if you want to try it they are:
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