The momentary power on when power is introduced is a self test that it does.  This is normal for numerous Dell PC's.
Hello there. When I plug in the pc (optiplex 3020) it seems to be working instantly and stops. Press the button again, it works normally. I write with the help of a translator. You can see the problem exactly when you watch the video on the link.  [URL] 
When I plug in the pc it seems to be working instantly and stops.