@korttoma Ah, okay, thank you. I'll give it a try.
Maybe connecting a Decoupling-Capacitor could also fix this problem?

@korttoma
I'm powering this with 6 x AA Batteries into the RAW Input from the Arduino and the 5v output from Arduino to the Relay.

I tried the relay sketch on my arduino mini pro (5v), step down converter for the NRF module and a two channel relay. It's conecte to a Raspi GW wirh the 2.0.0 beta and FHEM as controller. The switching works as expexted. but the relay switches continuisly on and off on its own after a certain time. I tried removing the loop-block but with no effort. Is there anything i can do to get a stable state? Thanks