[lubuntu] When X freezes but VNC doesn't...

I'm often running a simultaneous VNC window alongside a regular native X session. From time to time, and with no rhyme or reason that I can detect, my X window session hangs, producing a black screen. The keyboard isn't frozen (I can see the caps-lock go on), but hitting Alt-F4 or Cntl-Alt-F4 doesn't bring me to a terminal.

I'm talking about weeks or months...it's not common!

The odd thing is that my VNC session still is fine, and I can access everything fine. But to get back to a working X session, I've had to always close everything out and hard power down the computer. Then reboot.

Is there a way to just type something via the command line that will keep the VNC session going and just refresh the X window?

Thank you!!!

(I'm on Lubuntu 14.04, LTS)