After several months of usage without any random and self-initiated reboots, I experienced the first ones after having changed my mobile contract and with it my second SIM card last week.
For the record, I use the FP OSOS, which means that I could not upgrade the OS for now - so no idea about the update. I did not exchange the SIM in my first slot and it was always set to 2G only. With my very old second SIM (at least 7 years old, also set to 2G since I did not have any mobile plan anyway), I never ever experienced any non-desired reboots since I got my FP2 just before New Year.
After exchanging my SIM, I still did not have any reboot - retrospectively seen - up to when I changed its setting to 4G preferred etc. Then, I experienced up to 15 completely random reboots per day (maybe more if I had always monitored) while using and while not using the phone.
So, my first idea was that the SIM was the culprit since I never experienced any such reboots before changing the SIM. I was a little reluctant to go and change it, however, since it was a brand new one and since my provider (O2 Germany) charges 25 € for an exchange…
Next, I read this whole thread - took me some time - and read that some people had less problems with 4G disabled.
Therefore, since yesterday afternoon I changed the setting for my second SIM to 3G (preferred)/2G - and up to now I did not experience any more reboots (with and without WiFi, during use or in stand-by) and I am kind of confident that it will stay like this. [I will update in case this is not the case.]
So, to my eyes, at least for me, the culprit clearly is the 4G setting, even though I can’t verify it with my old SIM because it has been deactivated after the exchange.
I don’t know either if this has to do with software or hardware but it may give a hint to the developers (@keesj, @borjan) I hope.
Further, I am not sure if this problem has been amended by the update as I pointed out before, I will check once the FP OSOS is officially released - presumably with the update Hope this will be soon.
P.S.: It took me some time to find the location of the setting again, so for all those who do not know either: You need to go to Settings -> Data usage -> 3 dots in the upper right corner -> Mobile networks -> Preferred network type.
Edit: Update: I did not experience any random unwanted reboots since my original post up to now (9 days) and do not expect them to reappear. Now, I am about to test the 4G setting with the FP OSOS beta.
Edit 2: No random reboots any more in 4G setting with the new FP Open OS.