nvidia - Unity-greeter black screen on 13.10

I've recently installed ubuntu 13.10 64bit on a desktop (UEFI mode). After the plymouth splash i get a black screen. After some experiments I found a temporary solution:

• switch back to tty1 and login


• sudo unity-greeter


• get an awful error: (unity-greeter:2109): Gtk-WARNING **: cannot open display: Failed to launch bus: Failed to launch session bus


• sudo reboot


• after reboot unity-greeter starts up just fine until logoff


• (after graphical login a procedure to report an Xorg crash starts)

I have an Nvidia graphics card, I tried all drivers (nouveau, currunt, experimental, xorg-edgers etc) but there was no change (with the exception of the xorg-edgers that always gave the corrupted configuration prompt, which, sadly, didn't solve a thing).

Any help/suggestion is appreciated :)

EDIT: After some experiments (see comments to answers) the problems came to be more random. At the moment it seems not to present itself when there is an Xorg problem (apport starting just after login) but sometimes I get a clean start even when there is no error to report. I think there's a link between "having an Xorg error" and "starting unity-greeter/lightdm correctly" because I tried using a faulty mouse (pointer stuck at boot with both win7 and ubuntu) and in that case unity-greeter/lightdm always starts (but of course then I have to disconnect the faulty mouse...).

I was asked for logs: I'll provide them the next time the problem presents itself (I'll try some reboots but I don't know when it will happen again :( ).

I was also asked for info about hardware and configuration:

• lshw relevant info:

  
  
  

  *-display
              description: VGA compatible controller
              product: GF116 [GeForce GT 640 OEM]
              vendor: NVIDIA Corporation
              physical id: 0
  
              bus info: pci@0000:01:00.0
              version: a1
              width: 64 bits
              clock: 33MHz
              capabilities: vga_controller bus_master cap_list rom
              configuration: driver=nvidia latency=0
              resources: irq:89 memory:fc000000-fdffffff memory:d0000000-d7ffffff memory:d8000000-dbffffff ioport:e000(size=128) memory:fe000000-fe07ffff
  




• lspci -v relevant info:

  
  
  
  

  01:00.0 VGA compatible controller: NVIDIA Corporation GF116 [GeForce GT 640 OEM] (rev a1) (prog-if 00 [VGA controller])
      Subsystem: Micro-Star International Co., Ltd. Device 2319
      Flags: bus master, fast devsel, latency 0, IRQ 89
      Memory at fc000000 (32-bit, non-prefetchable) [size=32M]
      Memory at d0000000 (64-bit, prefetchable) [size=128M]
      Memory at d8000000 (64-bit, prefetchable) [size=64M]
      I/O ports at e000 [size=128]
      [virtual] Expansion ROM at fe000000 [disabled] [size=512K]
      Capabilities: 
      Kernel driver in use: nvidia
  
  




• Xorg version: 7.7 (apt-cache show xserver-xorg | grep Version gives: Version: 1:7.7+1ubuntu6)



• Nvidia driver version: 331.20 (from x-swat ppa)


• uname -r: 3.11.0-15-generic

I can't think of other info that could be needed, if you can just say so :)

EDIT: it has been 3 days since the last time the error presented. Nothing changed in the system (at lest not by my doing) so I have no idea of what caused it and what solved it. Thanks to everybody that helped, I'll post updates if (and when) I'll have black screen again...