MacBook Pro re-starts itself - with grey screen message - 4 times in a month with grey screen message

I have a MacBook Pro 15-inch Mid 2012 15.4"/2.7/2X4GB/1TB/SD machine which I am very much satisfied with - with almost no hassle. It has been in for service two times recently in which I got a new hard disk (in November 2014) and a new motherboard (now in April 2015) because of some random and smaller issues. I got this covered with my AppleCare. After I got it back last time in April 2015 (2nd. time) I started to experience that it re-started itself four times - during short period of a month (1 month and two days to be exact) before I decided to take it in for service again. The re-start goes to a grey screen which contain a message in different languages. This did not happen until I got it back from last service in April 2015. This restart has not happened because of any updates, and it happens just suddenly without any warning or reason (that I know of). The last (and 4th. before taking it in for service) it happened while I was watching a Facebook uploaded video (not YouTube or outside linked) which automatically starts when scrolling down the Facebook newsfeed - and this was in the Safari web browser which I am now only using after my last (April 2015) service/repair.

My MacBook was not heated, and I did not have many programs open. Those few open/active programs had no in activity, and I do not consider them as "heavy" programs.

- Safari (which I was currently using at the time it happened)
- Finder (active with dot below the icon, but no activity)

- email (active with dot below the icon)
- VLC player (active with dot below the icon, but no activity)
- one more program which I forgot what it was (also active with dot under the icon, but no activity).

What happens is the following: My MacBook switch itself off completely, then comes a gray screen that has a message in English and other languages, it pauses (for me to read the message), and then after a few seconds it restart it themselves (if I do not touch any buttons) - and what a bit weird is that all the programs I have had open before restarts so I am back to where I was. I do not get any alert messages like the "You shut down your computer because of a problem." or the "Report to Apple" - which usually happens when this type of error occurs.

I think this behavior seems a bit out of the ordinary since my MacBook is not heated, or I run any heavy programs - and the frequency of four times in a month's time is a bit too much.

My MacBook has now been checked and tested at the Premium Authorized Apple service place (same as the other times I have had it in for checkup and repair) for little over two week's time now. They tell me they have not found any error message in the log in sonsole, and they also done tests and tried to push it to the limit without it having this shut down/restart. They want to keep it for observation for some extra time, and I hope they find out exactly why this happens and that they of course fixes this.
It might be a "kernel panic" - but then again, it does not have the display of the message "Your computer restarted because of a problem." or display a message "You shut down your computer because of a problem." - and nor does it have the "Report to Apple".

Let me also add that I have a two 2 Terra AirPort Time Capsule in a network (one flat 2012, and one "high tower" 2015) that also my WiFi internet connection goes through.

1) Is there anyone here who has experienced something similar in such a short time (frequent restarts up to 3-4 times within approximately one month period) ?
2) What is - or what can be - the reason (why) this is happening (and why so often) ?
3) How can it be fixed ?

What worries me is that they are not able to see and experience this - and that they cannot fix it....