To me this seems most likely to be a case of a “loose” battery.
Here’s the troubleshooting “tool” from Fairphone:
https://support.fairphone.com/hc/en-us/articles/115001041206-Troubleshoot-your-Fairphone-2-issue
Select “Phone reboots”
Then select “There’s something else wrong”
Next you will see a video with a test for the battery frame.
If your phone reboots when you try this at home, select “Yes”
!Attention! take a video of this test, as you will have to send this video in.
Then contact support and you will find help (eventually 
).
You might have to be a bit stubborn.
My phone became virtually useless, as it rebooted or turned off all the time.
When I contacted support, they concluded, that my battery-frame is a bit large. That’s right and the troubleshooting tool already said so.
Strange enough they said, that the module would have to be replaced and that I should buy the module online and do so myself. If I would not feel up to it, I of course could send the phone in.
This left me wondering which of the modules in the shop I should get to solve my issue:
https://shop.fairphone.com/en/spare-parts/filter/choose-your-fairphone/fairphone-2
The (really expensive) core module - that might include the battery frame - is even not available in the FP shop (only with the German reseller vireo).
Therefore I answered, that they can’t be serious, as I see no module that might do the trick. Furthermore I said, that this problem is widely discussed and quite regular on this forum and that even the troubleshooting tool states, that the “battery frame of your device is slightly bigger than it should”, marking it explicitly as a production issue. Btw. what could I have done to make this frame bigger or to widen it by regular use of the phone?
I added, that I would’nt have coped with random reboots but would have contacted them before the end of warranty, should I have realized it could become so bad, I can not use my phone any longer.
In the end, they offered to send me a tiny self-adhesive plastic part, that is easy to assemble for free.
That did the trick! My phone works just perfectly fine and without any further reboot since then.
It’s just that they should have offered that solution right away. I even would have paid for it.
The initial reply was a complete fail, especially, when you did everything according to their instructions and supplying a video required by them, that illustrates the issue quite well.
All in all, I am happy again and still satisfied with Fairphone - the company. 