Sony vaio cr353 shuts down randomly?

My vaio Laptop VGN cr353 is shutting down randomly. sometime it even shuts down during start up. it shuts down unexpectedly without any warning as if i'm holding the power key for few seconds. After I turn the laptop on again, I don't get any error or any other messages, and the windows just start normally. I think it's not an overheating problem, because sometime it shuts down after only 5 minutes, and sometimes after 2 or 3 hours. my battery is good. oh, another thing, this happens after i replaced win vista premium (that comes with the laptop) with win 7 ultimate. but because i think it doesn't have anything to to with it, i didn't change it back to win vista. (i could be wrong). i found that some people that uses the same type of laptop (vaio cr353) experience the same problem, and they also couldn't found any solution. could it be the win 7? maybe a mal function hardware? Another thing, i live in Indonesia, (in north sumatra), and here we don't get good services from the shop provider. so i really would appreciate a good answer from here. thanks in advance.