Hello Jens Albrecht,
Before we can determine what the issue may be, we will need a little more information. With the inconsistency of the video stream, there can be many different reasons as to what is causing this.
1. What cables do you have connected to the audio/video source?
2. When you are not receiving video, is it for all the channels or just a select few?
3. When there is no video, are you receiving audio?
Awaiting your reply,
Sling Media Moderation Team