unable to log back into session after screen locks

My computer ruing 14.04 has started to develop a strange problem. If I leave the computer idle long enough for the computer to lock the session and suspend the display, I am unable to get back into my session. The display will wake back up and the display manager will come up. I can type my password back in, but then the screen goes black. There is a cursor that moves, but it's jumpy. I've tried using Ctrl-Alt-F1, but the terminal doesn't come up. I can still ssh into my computer and kill lxsession, in which case I get back to the display manager and can start a new session. I've had this happen with Xfce and LXDE. I haven't tried any other desktop environments. I'm not sure what has changed lately to cause this, as the computer has been very stable with 14.04 for almost 2 years now. Any suggestions for debugging?