[SFML] Dziwne skoki FPS

void cPlayer::update()
{
    sf::Mouse mouse;
    sf::Vector2i mousePos = mouse.getPosition( app );
    rotation =-( atan2( mousePos.x -( position.x - screen.x ), mousePos.y -( position.y - screen.y ) ) ) / 3.14 * 180 + 180;
    sf::Keyboard keyboard;
    if( keyboard.isKeyPressed( keyboard.W ) )
    {
        double speed = 150 * appClock.getElapsedTime().asSeconds(); //Szybkoœæ
        move( sf::Vector2i( sin( rotation * M_PI / 180 ) * speed, - cos( rotation * M_PI / 180 ) * speed ) );
        screen.move( sf::Vector2i( sin( rotation * M_PI / 180 ) * speed, - cos( rotation * M_PI / 180 ) * speed ) );
    }
    if( keyboard.isKeyPressed( keyboard.S ) )
    {
        double speed = 1.5; //Szybkoœæ chodzenia do ty³u
        move( sf::Vector2i( - sin( rotation * M_PI / 180 ) * speed, cos( rotation * M_PI / 180 ) * speed ) );
        screen.move( sf::Vector2i( - sin( rotation * M_PI / 180 ) * speed, cos( rotation * M_PI / 180 ) * speed ) );
    }
    appClock.restart();
}

void cEntity::draw()
{
    sf::Sprite spr;
    spr.setOrigin( originX, originY );
    spr.setTexture( eTextures.textures[ textureID ] );
    spr.setPosition( position.x - screen.x, position.y - screen.y );
    spr.setRotation( rotation );
    app.draw( spr );
}

appClock.restart();