Deus Ex Machina: Leibniz Contra Malebranche

I have been searching the 'Net and various databases such as JSTOR without success for a good article on deus ex machina objections in philosophy.  What exactly is a deus ex machina (DEM)?  When one taxes a theory or an explanatory posit with DEM, what exactly is one alleging?  How does a DEM differ from a legitimate philosophical explanation that invokes divine or some other nonnaturalistic agency?  Since it is presumably the case that not every recourse to divine agency in philosophical theories is a DEM, what exactly distinguishes legitimate recourse to divine agency from DEM? Does anyone have any references for me?  Herewith, some preliminary exploratory notes on deus ex machina.