If you've already tried all the buffer settings etc.. then you should call Cubase. You've got free tech support as long as you registered the software.

I've run into similar problems myself, but I'm on a Mac, so I don't know if it applies to your situation. Try using virtual memory and doubling the size of whatever ram you have. You'll get a warning that Cubase doesn't work well with virtual memory but just ignore it.
When you go to mix down, turn off the virtual memory so you get the screen draw speed back.