How to fix the cursor in First Person Mode?

local plr = game.Players.LocalPlayer
local cam = game.workspace.CurrentCamera
local chr = nil


chr = character
plr.CameraMode = 1

while false do
    wait()
    cam.CameraType = 0
    cam.CoordinateFrame = CFrame.new (200,10,0)
    cam.Focus = CFrame.new (200,7,500)
    cam.FieldOfView = 20
end

When I do this, and enter the cursor is constantly bouncing around. Is there anyway I can fix this?

I'm new to this, so I have no idea how that Lua button works.