The reason your sprite gets jittery is because your collision masks are inconsistent and because of that the collision mask keeps changing, which causes the sprites states to change. In one second the collision mask is not touching anything, making the sprite change to either jumping or falling.
I downloaded your cap and took a look at it, and made some changes to it (seems that the one I dl'ed is different/older than the one I get from your link now though, so keep that in mind)
There's two layouts in it. One where I just changed the hotspot of your sprite and another where I use another sprite, which is just a box, for the platform behaviour and control the animations of the character with events instead. I, and I think a lot of others, prefer doing it this way because although it's quick to get started using the animation tags, you'll soon want more control over the animations as your games get more complex.
I also noticed that you've set the "Walking/Running" as a sub-animation to "Jumping" in the latest cap. I'm not too experienced with sub-animations so I'm not entirely sure how to work with them, but I'm pretty sure that having walking as a sub-animation to jumping is something you don't want. Maybe someone knowledgable about the matter can chip in?