Well I will not count this as an aswer, as it does not solve your issue. Alas, I do not have enough reputation (which is understandable) to add comments. From my little knowledge on Lubuntu (well, Linux) I would suggest the following:
sudo apt-get remove chromium
Then follow up with an update:
sudo apt-get update
Finally, re-install the app with:
sudo apt-get install chromium
Failing that, would you be able to test the same processese whilst booting from a Live USB? That may help identify if it is an issue with your install, or hardware.
Adding to the above (this is an "updated"), if all works well running via the USB, then chances are it has something to do with the configuration of the OS. It should do the same on the LiveUSB (as it uses the same hardware).
It may just be the software, either Lubunutu or Chromium. Sorry I could be of no real assistance.