Please provide a little more detail about the problem.
Does this happen while you are using it? ...or only while in sleep mode?
Does it power down completely, or merely crash and reboot? (You can tell if the device has actually powered down by pressing the power button for 3 seconds. If the tablet vibrates in about 1-2 seconds after you release from the 3 second press, then it was powered off. If it does not vibrate after that 3 second key press, and takes two separate 5 second power button presses to boot, then the device is likely hung.)
The Chrome Beta browser was causing random reboots of my a500 until I uninstalled it. If the tablet is crashing and booting when you run Chrome, try removing that. This can be caused by other apps also.
If it just powers down without any warning and does not boot, then you could have a battery connection problem.
