Go into F2 BIOS Power Management settings.  Make sure nothing is set to automatically wake it up, like when power is restored for example.
I have a Dell Optiplex 755, everything runs fine, but for some odd reason, the computer powers on by itself and shuts down less than 5 seconds later. It’s a continuous repeat. Nothing shows on the monitor or anything. It just powers on and then off. It happens every 10-20 minutes  I don’t understand why it’s doing this. It started doing this when the CMOS battery said low voltage, well I replaced it with a 2032 energizer and it still happens. Everything works fine, I can use it but it’s just this odd issue after I turn it off.  what could be causing this? Everything is fine it works 100%, it’s just this odd flaw. 
Optiplex 755 Random power issue?