Check whether the Source item in the TRIG MODE menu is in conformity with the signal channel used in the practical application.
Check on the trigger Type item: The common signal chooses the Edge trigger mode for Type and the video signal the Video. Only if a proper trigger mode is applied, the wave form can be displayed steadily.
Try to change the trigger coupling into the high frequency suppress and the low frequency suppress to smooth the high frequency or low frequency noise triggered by the interference.