Faced the same issue. It started a year ago. I tried a lot of things without success. However about 2 months ago I decided that it can not be wors and disassembly the cambera (it is hero 4 black) completly. I clean up all contacts, used new thermal paste for CPU and other chips and then it started working again without any problem. But yesterday it started to freez again. So I did the same things but when assembly it back I forgot to plug the sensor. And it freez. After plugging the sensor and checked its contacts the camera started working wll again. So I think the problem with freezing is in sensor or its ribbon (cause when I did this first time 2 months ago I saw that the sensor was not so strong plugged). Try to unplug camera's sensor, clean up contacts and make sure it is plugged good and its ribbon is not damaged.