It seems high temperature causes this unexpected shut down.
Did you use the notebook on the table?
There should be enough free space to keep the air circulating.
Furthermore you should check if the cooling fans are not clogged by dust or debris.
This might affect the air circulation too… I clean my fans from time to time and this keeps the temperature low.
What graphic card driver do you use?
You should use the Toshiba one because such driver is modified for mobile usage… the other drivers don’t support an GPU overheating protection and this might cause such unexpected shutdowns.