Trembling Sprites cocos2dx 3.0 - c++

Question

I notice Sprite trembling while moving down with acceleration.. here is the whole code:
I am testing this code with iPhone 3gs, any guesses why?
TestScene.h :
class TestScene : public Layer {
public:
....
virtual void update(float delta);
float spawnTime = 0.9;
float untilNextSpawn = 0;
virtual void spawn();
float gameSpeed = 200;
float gameAcceleration = 10;
Vector<Sprite*> objects;
};
TestScene.cpp :
void TestScene::update(float delta) {
untilNextSpawn -= delta;
if (untilNextSpawn <= 0) {
spawn();
untilNextSpawn = spawnTime;
}
gameSpeed += gameAcceleration * delta;
for (int i = 0; i < objects.size(); i++) {
Sprite* t = objects.at(i);
if (t->getPosition().y <= 0)
t->setVisible(false);
else
t->setPositionY(t->getPositionY() - gameSpeed * delta);
}
}
void TestScene::spawn() {
Sprite* sprite = NULL;
for (int i = 0; i < objects.size(); i++) {
Sprite* t = objects.at(i);
if (!t->isVisible()) {
sprite = t; break;
}
}
if (!sprite) {
sprite = Sprite::create("prefab_button.png");
this->addChild(sprite);
objects.pushBack(sprite);
}
sprite->setVisible(true);
sprite->setPosition(Point(getContentSize().width / 2, getContentSize().height + 100));
}
prefab_button.png : http://i.imgur.com/oljgyZn.png
TestScene.h : http://pastebin.com/f9sSTLgp
TestScene.cpp : http://pastebin.com/siipLzx6
And here's video: https://www.youtube.com/watch?v=PYfzUnSBQ7M