After a year and a half of no problems, my MacBook has started randomly rebooting itself and spiraling into a reboot cycle. Example: after using my computer for an hour, it will reboot, operate normally for 10 minutes, then randomly reboot again and only operate normally for 5 minutes. The decreasing "normal operating time" will continue until it's rebooting while still displaying the gray boot screen (yikes!) and never successfully fully reboot. No new software installs (other than system updates - everything current) and when the problem starts I'm only running Word and Safari. At first I thought it might be a heat issue, but I downloaded a temp. monitoring widget, got the CPU temp cranked up, and couldn't replicate the problem. I took it into the Genius Bar yesterday and had them check it out (didn't find any hardware problems; and they did an archive install). Seemed to fix the problem, but it started again today. Help! Haven't found any similar posts... much thanks.