This is on my other 5,1 that only sees windows 10 on bootcamp ( my wife uses it, not a OS X fan). This mac hasn't been touched for about a year and is only used for msn.com & email. If my memory serves me correctly it's has Lion on it but again haven't checked. It's running 8gigs of memory & 1TB hdd. Video card is a radeon 7970. Now for the problem.....it shuts itself off within 30mins or so of inactivity. As long as you're using it, it will run flawlessly. Only shuts down with no activity. Just started happening last couple of days. Mac is spotless inside & out. Nothing, and I mean nothing has been changed in a year or so. I did throw the nvidia 120 back in it to see if it was the card but does the same thing. I know someone will say "PS" but will run all day in use, so would amaze me it that's what it is. Any one had this problem & found the problem?