Maintaining altitude problem

#1

Got a px4 with the original gps, 20A esc 390kv motors, 16x5.5 carbon fiber propellers, 3000g total weight
During flight in position mode it loses altitude randomly and have a hard time maintaining the target altitude. I attached a graphic.https://photos.app.goo.gl/bhwUuSvn3c9JEVbg9