Random Shutdown on Macbook Pro (15" mid 2014)

i have a Macbook Pro (Retina, 15', Mitte 2014) with El Capitan installed (10.11.6 (15G1004)).

It keeps randomly shutting down. This happens like this:

1. Screen turns black (does not turn off, logo-light is still on)

2. Fans accelerate,

3. After 5-10 secods, it turns off (screen & apple-logo turns off)

4. i can boot normally. no crash report.

I was not yet able to reproduce the problem. ive tried a lot:

- happens with or without power cord plugged in

- resetting the smc does not change anything

- reinstalling OSX does not change anything

- it feels like it happens mostly when having high CPU Usage, but running a benchmark (cinebench) to put pressure on the CPU does not automatically shut it down

- also happens with no CPU Usage at all (but fewer times)

What diagnostics can i provide to give you more detail?

What monitor tools are available to get more information about what happens?

Anybody encountered this... ever?

I'd appreciate your help!