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For troubleshooting on this, take a look at this article:
Troubleshooting wireless mouse and keyboard issues
For more information on this, take a look at this article:
Wireless input devices: Bluetooth frequently asked questions
Hope this helps,
When I first got a Magic Mouse I had the same problem. This 'fix' worked for me - make it a 'Favourite':
System Preferences>Bluetooth, select your BT Mouse on the left and then at the bottom of the window click on the 'cog' and select 'Show more info'. You should then get the option to make the mouse a favourite - select that and test.
Another tip that has helped a few is to put a very thin piece of card or paper into the battery compartment - just enough to make the batteries more secure. This seems to help ensure that the connections are more secure and less likely to momentarily disengage.