Q: Unstable magic mouse

My Magic mouse is really difficult to use now, probably coincidental with the upgrade to El Capitan. When I select a cell in a spreadsheet, the whole page moves around all the time, and it is really difficult to get an exact 'hit' on a cell. I was trying to live with it, but just  made a really serious error.

On simple text it is fine, quite stable. Using it on Mail though, is just as hard in that when you try to select the mail file, it constantly moves, showing 'mark as unread' or worse 'Trash'.

I have tried adjusting the mouse speed down, but it makes no difference.

I think I read that other users were experiencing similar, and wonder if there is a fix.

Thanks.