I agree with the majority on this problem.  Also check the resolution quality of each actor/prop in your scene.  Too many hi-res texturing can choke the system.  Lighting is really a big issue, and the fewer you rely on the better.  Maybe using an overall IBL to fill the scene is the answer.  It will illuminate everything and reduce the dependence of other light sources.  Or render in three stages, foreground, middleground and background, and then reassemble the entire scene in Photoshop or PSP by layering each render over each other.  I have successfully created renders in such a matter when I felt the system was too overwhelmed by the volume of objects in the scene.