rMBP keeps shutting off

[3lite]

rMBP 13" Late 2012
i5, 256 SSD

Examples:

1. When I go to bed and leave the macbook on (I don't put it to sleep or anything, just leave it there), when I wake up about 8 hours later, the macbook has shut off and to turn it back on I have to hold down Shift+Ctrl+Option+Power key simultaneously to turn it back on.

2. When the macbook is not plugged in, it will shut itself off randomly as I am using it. Again, I have to Shift+Ctrl+Option+Power key simultaneously to turn it back on again.

3. When the macbook is not plugged in, at times, it will just freeze as I am using it. Again, I have to Shift+Ctrl+Option+Power key simultaneously to turn it back on.

I have noticed #2 and #3 happens a lot more frequently especially when I am connected to a Wifi away from home at a public space (school, Starbucks, Library, etc.).

Solutions I have tried:

1. Clean install of el Capitan
2. Restored macbook
3. Reset PRAM, SMC, etc.

Does anyone know what my issue is or how to fix it? I cannot afford to buy a new macbook.

Thanks and please help.

 

[Gav2k]

Run the Apple hardware test?

 

[T5BRICK]

Does anyone know what my issue is or how to fix it?

Run AHT as suggested, or take it into Apple for diagnostics.