Why is this head rotation not working?

So here is the run down. I'm trying to have the head rotate up and down by the neck according to the mouse position, but its acting funky.

Here is the code:

Neck.C0 = CFrame.new(0,1.5,0) *  CFrame.Angles(math.sin((Mouse.hit.p - Mouse.Origin.p).unit.y), 0, 0)

You can see what its doing here: http://www.roblox.com/games/296695874/SS-Development

Please help, Ive tried everything I know to do.