Old Apple wireless keyboard not working with new Mini

I got this Apple wireless keyboard in 2003, model A1016.

It pairs with my new Mac Mini running 10.10.3 but after a couple of hours gets really flaky. It will have a horrible lag between keystroke and appearance on screen, or page up will push a Safari page all the way to the bottom, it will inexplicably throw in a bunch of blank spaces or it will bring up a balloon to select a foreign character.

It works fine on my late 07 Macbook running 10.7.5. An older Macally USB KB works fine with the Mini. but it lacks volume control and eject button.

The Mini also is running a new Magic Mouse.

What's going on? II've tried replacing batteries, repeatedly shutting down, removing KB batteries and re-pairing but it still gets really flaky.