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You may also find different results depending on the location where the user is when they do a search. It's also possible to have some penalty or filter applied on google.com and not on google.fr. Try a Site Search [webmasterworld.com] for Google geo-location, ccTLD and so on.
I have nothing I could add to what members are already sharing in those threads.
I'm experiencing the same issue as you except that my site used to rank top 10 for a lot phrases in google.com, but now only a handful still rank.
I found that I still rank quite well in other TLDs. My gut feeling is that I have been penalized in the .com
I have tried to find a way out but nothing has worked yet.
Let me know if you have any luck.