Hey guys I am using a refurbished Macbook purchased in late 2011, its running on Mac OS-X version 10.6.8. I had to change my laptop's old hard drive to a new one after I experienced the 'grey screen of death' two weeks ago. I also bought a new battery for my laptop as the old one was starting to go bad. Even after that, I am experiencing random shutdowns. This happens only when there is high performance involved - when browsing the Internet and I have a lot of Safari tabs open, or when I am transferring files from my portable hard disk drive. Any suggestions?