Hardware -problem, loose connection? Phone restarts frequently

My smartphone frequently blacks outs and either restarts immediately or doesn’t react untill I boot it up again. The problem is reproducible as the phone shuts off when I lightly tap it on the table as if I wanted to just put it down. (so, very light “shaking” already does the trick)

Either way I just noticed that it shuts down laying on the table not being touched at all, too.

Could it be some loose connection or a battery problem? I just purchased a new battery but it won’t arrrive until Thursday. Current battery is on 100%, loading was not a problem or strange at all. The phone is currently plugged in, too, but the problem remains.