Haya okey this is what i usually do to get around this issue.under your rendering sub menue go to the render dropdown menu and go to the batch render option.
Now be sure to uncheck you auto memory limit. and tipe in a reasonable amount of ram.
The reason mental ray crashes, is becuase people dont seem to realize the it is trying to take as much of the system resources to do the rendering.. always try to give your machine some room for a sestem swop file to run by its own.
Hope that help you out.Also rather render with the batch renderer. cuz its a non visual way of rendering so you give more room for mental ray to play around.