Error Compass Variance and Plane Goes off Course

During some recent flights with my Skywalker 1830 fixed wing, I got “Error Compass Variance” and in one situation the aircraft started heading well off it’s intended path. I have the GPS unit mounted quite well clear of everything else, as shown in the photo. Any thoughts? Will it help if I disable the internal compass? I can put the bin file up if that helps.
Bin file can be found here: