I have been running into this too, and I thought it was the lights (custom Lights). Someone suggested I look into materials and though it killed the effect I was looking for in the render I removed some of the nodes in the material room and it rendered. Now, I do not think it was taxing my machine, but I do think something was/is funky with the materials/nodes. When I can get time to plug and remove each node one at a time or add them back one at a time I will let everyone know, but you might want to try disconnecting some nodes and see if it renders.

ratscloset
aka John