One of the rings may be machined slightly offset, and is installed reverse of what it should be. Examine to see if the chainring bolts are sitting recessed as they should (though not applicable on some French cranksets, particularly Stronglight).

Alternately, the chain might simply be too narrow for the purpose - is it a 9 speed chain, by any chance? Try putting on one of those wide Taya chains in its place.

-Kurt