Hopefully I can get some insight here, I'm at a loss. My MacBook Pro randomly reboots with no crash log/kernel panic. It tends to go in cycles, so once it reboots once, it will "chain reboot" on the grey screen repeatedly, usually not getting past the apple logo. Sometimes it will go without rebooting for a few hours, and sometimes it reboots every few seconds. It reboots in safe mode and during the AHT, although I was able to complete one short AHT, showing no problems. I've verified and repaired disk and permissions, no change. I hesitate to try a software update, because the nature of the reboots lead me to believe it may happen during install. I'm running 10.8.5 and have made no recent updates or changes, it was working fine yesterday, now it isn't. I rebuilt this computer from a "destroyed" macbook over a year ago, and it's been A-OK until yesterday. I have tried switching the battery and the issue still occurs. There is no error message, kernel panic notification, or crash log with any restart, when it actually boots into OSX, there is no indication that it wasn't shut down normally. Anyone have any clue? EDIT:: Zapping the PRAM 4 times did nothing, but apparently the 5th has done the trick, because I haven't had an issue for around 6 hours (longer than I have been able to go without issue thus far). I'll update if the problem persists.