If your straight flying isn't stuttery then more than anything it's probably because you are turning the aircraft too quickly. The slower the better with a pan or tilt, however your shutter speed setting is important for getting good quality video and I generally shoot in manual mode for this reason, occasionally shutter priority - but forget auto.
The rule of thumb is to have your shutter speed at twice (or a multiple) of your fps. So if shooting at @ 30fps (720 / 1080 / 4k) the shutter speed ideally wants to be 1/60 sec, shooting at 60fps it wants to be 1/120 sec. Shooting at 60fps will also give a smoother video because but takes more processing power. Too high a shutter speed can lead to a 'Jello' effect which isn't good.
Having set the fps and shutter then set the ISO for the correct exposure. With the X3 this often means you will need to use ND filters even at ISO 100 because the aperture has no control to reduce the exposure. Even at 100 ISO with the supplied ND filter I find the exposure can be too high so either use a stronger ND filter if you have one or reduce the shutter speed, keeping it as a multiple of the fps.
I suggest you try 720p@30 and then 1080p@30 with the shutter at 1/60 if you can and see what you get. No need to fly for this test, just place the aircraft on a table and pan at the same speed for both settings. Also try different pan speeds and see what difference it makes.
This link is worth reading for information on camera settings when shooting video
Just experiment - I'm sure you'll soon get it right.