Sometimes on both my Phantom 3 and Phantom 4, when I first take it out of the case, and I start flying, I noticed that the camera is slightly off and rotated maybe a few degrees. In my videos, they seem to be off. But after I fly awhile, it straightens itself out. Why? And how do I fix this issue? I don't always notice until I'm in the air and by that time, it's annoying to try to land it to fix it. I do it by hand and it still doesn't really maintain a level image. Thanks in advance for your help and comments!

For example, look at this video at the 1:45 mark

I've had the problem sometimes before too. I think that it comes from booting up at a location that isn't level. All the same, with the P4 (and I think the P3 as well) there is an easy fix that you can make after takeoff. Select the camera settings icon and select "Adjust Gimble Roll." From there, you can manually adjust the camera.

Hello lugomaidem, it would be better to post 2 short examples (one for P3 and the other for P4) here. Also, have you calibrated the IMU and gimbal on the horizontal platform?