You're moving independently and applying speed along the X axis and Y axis, but the player speed (I'm assuming) is the total speed.
For example, if the player speed is 1, and the angle is 45 degrees, the player will actually be moving at a speed of ~0.707 along both X and Y axis.
Your camera code here will move the camera as if the player was moving 1.0 on each axis, rather than 0.7 on each axis, which means it will overshoot, then undershoot to compensate, then overshoot again, etc... causing the shaking you're seeing.
If you want a camera that keeps the player rigidly in the center of the screen, it's much simpler to do this.. you can just set the camera position to the player position directly:
// not sure on the exact code here, too lazy to look up:
View.setPosition( Player.getPosition() );
Or if you want to keep the camera more "fluid" so the player isn't necessarily in the exact center all the time... it'd be better to get the player speed as a sf::Vector2f where you have independent X and Y speeds that you can use to move the camera.