Three things to do:
1. Check event log to see if that tells something about the cause.
2. Boot from a Linux disc or stick (free to make) to see if it's the same there.
3. Disable all programs in the startup tab of msconfig (pre-windows 10) or task manager (windows 10) and reboot. Any difference?
my laptop shuts down and then restarts by itself.i have done a clean install and this still happens.i am using windows 7.i am pretty sure this is not a overheat issue because it usually happens as soon as i turn my laptop on.also this is not happening on safe mode.