I believe I'm experiencing the same issue as you. In response to the other answers referring to the settings for your lid being closed, I am assuming that is not the issue. With my laptop, I have specifically turned off the option for turning off when the lid is closed and I STILL experience this problem.
I would say you've taken the right approach so far with cleaning the dust out and adding extra ventilation. I think heat may still be the cause of your problem. Try downloading one of those free programs that monitors the temperature of your CPU. Keep an eye on it and see if the temperature increases dramatically until it shuts itself off. I did this and I saw that my laptop is hitting over 170F right before it crashes out, so I'm assuming I need more cooling options.
Can anyone recommend a good free CPU temperature monitor?