Macbook Pro goes to shutting down / sleep randomly

Hello all,

I'm having trouble with my MacBook Pro going to sleep randomly. I will be working on the computer, and everything will go black. I can still here the computer running. When trying to wake it up from what seems like sleep, no buttons will work. The only thing I can do is do a hard shutdown, holding down power button, and then turning it back on.

This originally happened about 2 months ago, and only once. Since about 2 days ago, it has been happening about 2 times a day. It occurs when the temperature of the computer is low as well as when it is warm. I am not running too many programs when it happens. I have reset the PRAM.

Any ideas / comments would be appreciated and helpful. Here are my system specs:

15 inch Macbook Pro
2.4 GHz Intel Core i5
4 GB 1067 mHz DDR3
Mac OS X Lion 10.7

Please let me know if there is any other information I can provide to help diagnose the problem.

Thanks again.