I also have this model (toshiba satellite p755-s5320) with the exact same problem. It began to shut off unexpectedly about 4 months ago, I took it to the service and they told me the problem was in the heating system and also maybe the hard drive. They told me that they fixed the problem and it did begin to work normally about 2-3 months and now it again began to shut down, sometimes even only log off and sometimes start again by itself. I took it to the service again and after a week of diagnosis they now tell me that something is wrong with the motherboard and it is a common issue with this model. I do not know what to do know. Are there any ideas?