I would run paper path tests to check the printer is fine when printing duplex, if it is then I would look at the issue with Windows 7.
It could be co-incidence so it's worth printing paper path test of 10 sheets or so, enable duplex=on and check it works as expected, print the menu map to find the path test option.
When it jams, where is the media, what error codes do you have, is the media always in the same place and does the paper path test work fine ?