Thank you for your answer. I am sorry if it is a little bit confusing. I will try to upload a video where you can see the problem as soon as possible.
To your questions and suggestions: It is a "transferred" arc type. And the flicker is during pilot arc and transferred arc.
We have already checked all electrical connections and work clamps and cleaned everything (also the pipelines, valves, etc.). We have also checked the electrodes, power supply, gas quality, gas pressure, but found nothing. So we really run out of ideas...
Maybe one additional note: We have two automatic welding machines from two different manufacturer. Both worked without any problems and then out of a sudden the same described problem occurred on both machines.