This sounds like a foundation problem to me (I had a one car garage doing that and it was the foundation causing it) but you really should speak to an expert. There is one really nice man online where you can possibly get a free opinion. Be patient if it takes him awhile to answer. http://www.askthebuilder.com/
Click on the 'Ask Tim' button in the yellow strip towards the top of the web page. Good luck and let us know what you find out :-) He also sends out newsletters every now and then that you can sign up to receive. He's very personable and I would trust his advice.
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