I had the same issue with my own FP2, but I could easily erase the problems. With my FP2 it seemed to happen, when the battery was too loose and did not connect well enough to the phone itself. If this is your case, too, I recommend using https://forum.fairphone.com/t/fp2-reboots-on-mechanical-pressure/23010 . Just take a piece of paper, fold it so that it fits tight on the lower end of your battery into the case and look what happens.