I too had this problem and still do. I had Galaxy S LCD (GTi9003) which used to shut down on its own. Then I had Micromax A30 which also did the same thing. Sometimes, the phone would automatically reboot. Sometimes, it would just shut down and not restart. I tried everything I could :rooting, updating to latest firmware, downgrading to older firmware (to check if it is a bug in the firmware), resetting phone. I even used the root ability to wipe OS from the phone and did a "clean install of a new OS". Still, no result.The only thing I didn't do is install custom rom. My Galaxy S LCD got its motherboard spoilt just after 2 years. The micromax phone is quite new to say anything about it. So,I believe that it is a serious problem with the handset, mostly motherboard. In taht case, there is apparently nothing you can do. But, in case it is not motherboard, you have hopes. Try doing the only thing I didn't : installing custom ROMs. Also, try not to keep the phone in a case (if climate is hot) and try not keep your phone in "tight jeans", under pillows or other places that may exert pressure on the phone. If it doesn't work, it is a problem in your handset.