How to make billboard more crisper and sharper?

CesiumJS
1

Hi! I am currently developing a project using Cesium, and need your help on the things listed below. There two problems with visibility of billboards. The first problem is when we zoom in very close to billboard, billboard jumps to up and when we zoom out billboard returns back. I want to billboard doesn't jump when we zoom in and zoom out very close to it. And there is second problems the billboards don't look sharp and crisp. My billboards are images .png with transparent background and with text inside in it. And they are not sharp and crisp while we don't zoom in very close to billboard. Some additional information:
1. This link is how my billboard looks like: https://drive.google.com/file/d/0B8FAR4jw5j7DOEZwNzFoWW5IRTQ/view?usp=sharing
2. And this link how billboard must look like:
https://drive.google.com/file/d/0B8FAR4jw5j7DYWlLd3gtRmRodVU/view?usp=sharing
And about billboard jumping:
1. When we zoomed in but not so close https://drive.google.com/file/d/0B8FAR4jw5j7DYXBDRDlkV1BsZFU/view?usp=sharing
2. When we zoomed in close https://drive.google.com/file/d/0B8FAR4jw5j7DeTIyRGQ5Q2p2LXc/view?usp=sharing
3. When we zoomed in very close https://drive.google.com/file/d/0B8FAR4jw5j7DN1dQOE41VHBCa0E/view?usp=sharing
var positions = Cesium.Cartesian3.fromDegreesArray(elementCoords);
It must be like in 1. When we zoomed in but not so close

Here are example of my code:
//I use this code for find out the center of polygone and placed billboard //above of polygon and to the center of it
var positions = Cesium.Cartesian3.fromDegreesArray(elementCoords);
var bs = Cesium.BoundingSphere.fromPoints(positions);
var center = Cesium.Ellipsoid.WGS84.scaleToGeodeticSurface(bs.center);

polygon: {
hierarchy: Cesium.Cartesian3.fromDegreesArray(polygonProp.coords),
material: Cesium.Color[polygonProp.polygonColors.polygonColor].
          withAlpha(opacity),
},
position: center,
billboard: {
           image : polygonProp.polImage,
           eyeOffset : new Cesium.Cartesian3(0.0, polygonProp.max_height > 0 ? polygonProp.max_height : 2000, -100000),
           scaleByDistance : new Cesium.NearFarScalar(1.5e2, 2.0, 1.5e7, 0.5)
}
Could you please help me with solving these two issues?
What are the possible ways out?