X crashing randomly in lubuntu

I'm having a problem with the X session crashing randomly in lubuntu. I can't reproduce this on demand, but about once a day the screen goes completely black. When this happens, if I press keys on the keyboard it displays keyboard output. If I press ctrl-alt-f1 I can get into a text based console.

I have looked through the logs in /var/log but haven't been able to find the cause. Are there more logs I need to look through? Any ideas on how to troubleshoot/fix this? Below are the specs. Thanks!

-lubuntu 14.04.1 64-bit LTS
-OpenBox 3.5.2
-LXDE