Computer will shut down by itself while using computer?

Hello, let it cool good.

Get Temperature Monitor to see if it's heat related...

http://www.macupdate.com/info.php/id/12381/temperature-monitor

iStat Menus...

http://bjango.com/mac/istatmenus/

And/or iStat Pro...

http://www.islayer.com/apps/istatpro/

Also, open console in Applications>Utilities, check the system log for the date/time of the last problem for clues.

Reset the SMC.

1. Shut down the computer.
2. Unplug the computer's power cord.
3. Wait fifteen seconds.
4. Attach the computer's power cord.
5. Wait five seconds, then press the power button to turn on the computer.