I got a christmas gift 2-3 years ago : a magic mouse!! After several months of happy use, the bluetooth started having some down times... I checked everywhere on the web for answers, but found none that really helped except that one where the heat seems to affect the bluetooth circuit. After opening my macbook 2-3 times I figured that the bluetooth cable linking the motherboard and the BT board seemed to be the problem ergo my fix!
The repair went very well! No trouble at all except a couple of screws that are damaged a bit after the fourth opening of the laptop. Hopefully, it lasts for another 2 years (8 total!!) After that... probably retirement :)
Pressing on the keyboard around the center near the "U" key would often help to regain BT function for a while. I think it might have been a broken wire in the cable between the BT and mother boards. If this happens to you, the first hardware fix I would try is changing the MacBook Bluetooth Board!! The ifixit guide is already very good help for this and their parts are affordable and good!