Hi,
My pick is your motherboard is already faulty but try these steps to further support this theory.
- Remove the HDD and turn the laptop on see if it stays on, if it does HDD is already damaged.
- If even without the HDD the laptop still reboots by itself proceed with re-seating the CMOS battery
- Remove the CMOS battery for couple of second then put it back then turn the PC back on
- If all these steps results to the same issue it's actually is the motherboard that is faulty.
Here's also a support page from Sony that might have couple of troubleshooting steps that might help.
https
/esupport.sony.com/US/p/support-info.pl?info_id=773