It sounds like either the rotational angle is incorrect, or the front derailleur limit screws are not set correctly -- if the chain is coming all the way off, then just adjusting the cable tension with the barrel will likely not work as a temporary fix.
Look at the section on the limit screws; you can at least temporarily fix the problem by adjusting (tightening) the H / high limit screw, even if the rest of the setup is not optimized (derailleur height, rotation, limit screws, tension). On a newer Shimano derailleur, the high limit screw is usually marked "H" and the low limit "L."
http://www.parktool.com/repair/readhowto.asp?id=75





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