I get both of those a lot..

I do the following to correct it and it sometimes works.

1.  Shut down all other programs and unneeded processes.
2.  Save file and shut down C6.
3.  Reopen C6 and the file.
4.  Remove unused master shaders and objects.
5.  Set main assembly room display to wireframe.
6.  Reduce image resolution as well as quality settings a bit.
7.  Try to render again.