By shutting down do you mean a reboot or the same as would happen if you had a power cut ? If so I reckon you need to claim under the warranty assuming you have tried the usual power off at the mains for say 30 minutes and a reset to factory defaults. Could be an intermittent fault in the power supply unit.
To be honest I can’t remember what it does during a power cut?
All I know is it turns off and then I see Start System as it reboots.
I have tried what you suggest and it still happens. I am actually getting a replacement but just wanted to find out what could cause this in case the same thing happened with the new one.