Magic Keyboard /w numpad connection issues on 2017 iMac

Hi,

my new iMac is running since friday (18,2 mid 27" model). I am more than happy with it! Except: Since the beginning I have experienced several times, that after a sidden time all keyboard inputs get laggy (or even are repeated multiple times). E.g. when typing any key like "r" --> rrrrrrrrrrrrrrrrr... (it runs until I disconnect the keyboard in the bluetooth menu). After reconnect its working fine until the next "error".

Lates macOS installed (10.12.5).