Phone switches off and on randomly

hi there,
i bought my fairphone like 2 weeks ago. after the first day it already started to switch on and off while watching a video or using maps. I brought it back to the shop, they said it could possibly be because of an old sim card. So i changed both of my sim cards. But it still does it. Meaning exactly, I am watching a video for example and then suddently the phone stops, switches of and on as if i would have switched it off. Anyone familiar with that problem…?
Thanks so much in advance. I am so looking forward for this phone to work properly!

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