Particularly large background keeps snapping and is unable to move where I want it to

I've been trying to create a moving background (tunnel), and its meant to be quite fast paced so I've made a long background. But when I want to select the position of the background symbol for the last keyframe, it just snaps downwards and im unable to keep it how I like it. Is it because I'm moving it diagonally, because that's the perspective I need for the video or is it because the image is too big.

For example, since it kept snapping, I tried to avoid it by using the properties tab to create the position of the last keyframe of the motion tween and, I think the position is too far for the document, so it reverts back to the maximum number I could go.

(It doesnt render when i zoom out btw)

So does anybody know a solution to this?