[HELP} - Need stable loiter

For a project I am working on, we need a very stable loiter. I notice I have an AltErr and the plane is oscillating while in loiter, about 10-15 meters and speed is varying as well. I suspect it is a gain on the GPS, but not sure. What do you all need to be able to help?

Appreciate any insights.

posting a full log would be helpful for sure.

Here’s the log: