Strange Head/Neck Behavior, Only In-Game

Hello! So, I’m trying to make an avatar (using a Team Fortress 2 model, the Scout specifically) and I’m having an issue I’ve never had before: when walking backwards or side-to-side, the head jitters and bobbles, and when using GoGoLoco 1.8.3 to sit down, the head sinks into the neck along with the view point. This only happens in-game and not with the Avatar Emulator in Unity.

I’ve tried everything from changing the armature to removing GoGoLoco to see if it fixes the issue and nothing has worked so far. It’s driving me up a wall, lol.

If anyone has any ideas or suggestions, I’d be really grateful to hear them. Thank you!