Apple Users

I am willing to bet this is a setting matter, not hardware, but check your battery. Maybe it is dying. If the machine is under Apple Care you can probably get a new battery.

Click the Apple (upper left) > About this Mac > More Info > System Report > Power

At the top of the resulting screen you will see "Battery Information". Look down to the lines "Charge Information". The big number here is Full Charge Capacity. You want to be around 5000 mAh.

Under "Health Information" look to see what the cycle count is. Under 500 or so is probably perfectly okay. Apple says the batteries are good for 300 cycles, but I have had one go over 1200 and still be useable (reduced battery life, but useable). If you have a very low cycle count, that's not good either. Let the machine run down and then fully charge. Cycle it like this a few times a week until the count comes up a bit. This could revive a slowing battery. This is called "calibrating" the battery. The Apple web site has a some info about it.

Look further down that window to see the current settings. It might be a sleep setting and not a power setting.

Anyway, hope this helps.