I have a server/workstation that has been running smoothly for the last 4 years or so. The last night it started doing random reboots: screen goes black and the machine boots just like that. I've had 6 reboots so far, including one while the machine wasn't yet finished booting. Temperatures are fine. Smartctl tells me my drives are fine. I don't see anything strange in the kernel or system logs. How can I diagnose this besides taking the machine apart ?