HI,
Please do try these troubleshooting steps that may help in solving the issue.
- Start by doing a hard reboot, remove the battery and unplug its AC adapter then press and hold the power button for 20 seconds then test it again.
- If these doesn't work do remove the battery and test it if it would shut down by itself without it.
- You may need to clean the inside of your laptop and apply new thermal paste on both the GPU and CPU if it would still shut down by itself without the battery.
- Do also try a different AC adapter that has the same voltage and see if it will shut down by itself or not.
- Lastly, uninstall/reinstall the graphics card driver of your laptop.
- Go to Device Manager and uninstall the graphics driver.
- Check Programs and Features and uninstall anything related to your graphics card.
- Next is to download and install the latest graphics driver from the manufacturers site.
- Reboot your laptop once the latest driver has been installed.
- If all these will not work you may need to contact Toshiba Support if it's still under the Factory Warranty for a repair.