Its the Logic board. And the bad behaviour gets worse in cold conditions. Try in a warm summer/ environment: Hold down the start button, then plug in the mains cable, while holding down the start button a little longer - then let go of the start button... Wait .. sometimes it will start bij itself - or push the button once again. Nothing? Unplug the mains cable. wait a minute. (Try again on a warmer day!)

I'm still trying to get to the right solution...