Tracking data isn't consistent with spline outline when pasted into AE CC 2015

I’m editing a scene that was shot by attaching a gopro to the side mirror. I drew a spline around the headrest to track the bumps in an effort to replace the sky and keep the motion of the replacement sky (at least the “bumpiness” of the replacement sky) consistent. The spline outline in Mocha AE appears to be perfectly outlining the headrest for the entire track.

I then pasted the tracking info to a null in After Effects and parented the replacement sky to the null track. The tracking is great for the first 5 seconds and then suddenly moves to a much higher place on the screen. It then remains pretty accurate to the bumpiness of the car (albeit higher on the screen than it’s should be) but about 5 seconds later goes all over the place. Once again, when I play thru the tracking data in Mocha AE the spline stays with the headrest throughout the entire clip. I’ve tried retracking several times, and can’t seem to avoid this. I’ve also saved and restarted Mocha. Because the gopro is attached to the side mirror the headrest doesn’t move much at all throughout the clip. I’m not sure where all this extra movement is coming from. Thanks for any help!

Also, it’s probably worth mentioning that I tracked the headrest with only translation checked since I didn’t need scale, rotation, shear, or perspective.

Would you be able to send us the problem clip? This seems like a pretty straightforward track so i’m surprised it would move this much on export.