Computer Turning on Seemingly at Random

[frying_pan_cat]

Just an update: it hasn't turned itself back on in the last week. The only thing I changed was swapping out an old DVD-RW for a new Blu-Ray drive although I don't know if the old drive could have been the problem

 

[Parsley]

A simple suggestion but perhaps the power switch is shorting?

Try disconnecting the power button cable from the keyboard when you next turn it off for the evening and see if it still reboots perhaps.