Have you heard about the "piece of paper/card" trick? Some Fairphone 2 have batteries that are slightly too small and so any movement causes the battery to temporarily disconnet. So put a small (0.5cm) piece of paper at the bottom of the battery to keep it firmly in place.

I have had this problem too. The phone seems to get stuck into thinking that the battery is very low, even though it isn't - hence it keeps shutting down. What seems to sort it out is removing the battery for 10 mins (i.e. longer time) and then replacing it and fully charging it.