Magic mouse moves, zooms and changes position "magically"

Hello all, I use the magic mouse for a lot of time, but for already some 8/12 months it started to present a weird behavior, zooming by itself, moving the cursor up and down according to its own wish, changing the cursor position also at its own will, etc. As it was an old mouse, I assumed it was the age and decided to buy a new one, what I did some 7/10 days ago and gave the old one to my daughter. Well, for my surprise all problems the old one had, under my daughter's hand, just dissapeared and it now works perfectly.

Second big surprise, the new one, under my hand, shows exactly the same problems of the old one. It is just to take my hand from over the mouse and the "magic and autonomous" undesired movements stop immediately.

I concluded the problem it is not in the mouse, but maybe my hand and/or my iMac

Does anybody ever experienced a similar situation? Any hints will be very welcome

Thanks Paulo