Bluetooth connection instability with Magic Mouse on El Capitan
In System Preferences for Bluetooth you can check the box to "Show Bluetooth in menu bar." Then, hold down the option key and click the Bluetooth icon when paired with your device. You'll see a new option to "Create Diagnostics Report on the Desktop." This will give you more details to further debug or diagnose what's going on, particularly the files pmset_everything.txt, and com.apple.Bluetooth.plist in the section on BluetoothStats. My system displays 0 for all the stats except 1 for CommandTimeouts.