Not sure then, if your clean up doesn't work, try deleting all the render layers, then start again and make sure you've added all the objects into just one, thewn try rendering that a couple of times.

Apart from that the only other thing I can think of would be that you've got some objects selected before you start rendering, and you've got "render selected objects only" selected. That would make different objects render at different times depending what you have selected before you open up the render view.

If it's neither of those things, I'm lost!