That would indicate a hardware problem I think.
The only thing I could think of otherwise would be tweaking CPU settings to perhaps work around a performance or heat problem … e.g. with Kernel Adiutor or with LineageOS performance profiles.
If you happen to have fairphoneangels in your vicinity, they could perhaps help by swapping modules to find out whether a module is at fault, even if the expensive core module seems most likely to me.
Else … install one of Fairphone’s OSes and contactsupport …
… and quickly for a possible warranty case, if it was the end of 2016.
Can you give more details on that?
WiFi works fine for me.
Edit: Wait … does LineageOS not do the reboots?