Character arm joints "buckling" for no reason

Hey, this script that I’ve been running for a while now (a script to make a character look at the cursor) has suddenly started to exhibit some weird behavior, and I have absolutely no idea why considering the fact that it doesn’t even touch the arms of the character.

Here’s what the intended effect of the script is supposed to look like:
image

And here’s what the character looks like now:
image
See how the arms are “buckled” inwards? I don’t know why this happens, but I need to be able to fix it.

Script (to be ran in StarterGUI):

local RunService = game:GetService("RunService")

local Player = game.Players.LocalPlayer
local PlayerMouse = Player:GetMouse()

local Camera = workspace.CurrentCamera

local Character = Player.Character or Player.CharacterAdded:Wait()
local Head = Character:WaitForChild("Head")
local Neck = Head:WaitForChild("Neck")

local Torso = Character:WaitForChild("UpperTorso")
local Waist = Torso:WaitForChild("Waist")

local Humanoid = Character:WaitForChild("Humanoid")
local HumanoidRootPart = Character:WaitForChild("HumanoidRootPart")

local NeckOriginC0 = Neck.C0
local WaistOriginC0 = Waist.C0

Neck.MaxVelocity = 1/3

RunService.RenderStepped:Connect(function() 
	local CameraCFrame = Camera.CoordinateFrame

	if Character:FindFirstChild("UpperTorso") and Character:FindFirstChild("Head") then
		local TorsoLookVector = Torso.CFrame.lookVector
		local HeadPosition = Head.CFrame.p

		if Neck and Waist then
			if Camera.CameraSubject:IsDescendantOf(Character) or Camera.CameraSubject:IsDescendantOf(Player) then
				local Point = PlayerMouse.Hit.p

				local Distance = (Head.CFrame.p - Point).magnitude
				local Difference = Head.CFrame.Y - Point.Y

				Neck.C0 = Neck.C0:lerp(NeckOriginC0 * CFrame.Angles(-(math.atan(Difference / Distance) * 0.5), (((HeadPosition - Point).Unit):Cross(TorsoLookVector)).Y * 1, 0), 0.5 / 2)
				Waist.C0 = Waist.C0:lerp(WaistOriginC0 * CFrame.Angles(-(math.atan(Difference / Distance) * 0.5), (((HeadPosition - Point).Unit):Cross(TorsoLookVector)).Y * 0.5, 0), 0.5 / 2)
			end
		end
	end	
end)

Thanks.