Why does my script perform random errors when clicked in GUI?

[Edit 19/5/2018 V2]

This is my select the gamemode gui. I changed it into different bright colours to see the performance better.

It works perfectly. However, sometimes, it changes randomly and I do not know why. By watching the link below, you notice that the previous button does not transition efficiently. It does work but unexpectedly it glitches out. I cannot find anything wrong with it. Any ideas?

VideoLink

As you can see in the video, the runs perfectly on the first run but does random things after that. What is the problem here?

-- Declaration Section 
-- / Player Precedence 

    local Player = game:GetService("Players").LocalPlayer
    local PlayerGui = Player:WaitForChild("PlayerGui")
    local Gui = PlayerGui:FindFirstChild("Gui")

-- / List of Actual Frames 

    local frameList = {
        [1] = Gui.frame0, 
        [2] = Gui.frame1, 
        [3] = Gui.frame2, 
        [4] = Gui.frame3, 
        [5] = Gui.frame4, 
        [6] = Gui.frame5,
        [7] = Gui.frame6
    }

    local CurrentFrame = 1 

-- / Configuration for the movement of the frames

    local AnimationLength = 0.5
    local AnimationStyle = Enum.EasingStyle.Sine
    local AnimationDirection = Enum.EasingDirection.Out

-- / Buttons 

    local previousButton = Gui.Previous
    local nextButton = Gui.Next

-- Processing Section 

    local function ChangeFrames(MovingFramesArray)
        for i, FrameData in pairs(MovingFramesArray) do
            if FrameData == nil then return end
            FrameData[1]:TweenSizeAndPosition(FrameData[2] or FrameData[1].Size, FrameData[3] or FrameData[1].Position, AnimationDirection, AnimationStyle, AnimationLength)
        end
    end

    nextButton.MouseButton1Down:Connect(function(x, y)
		

        if not frameList[CurrentFrame + 1] then return end
        CurrentFrame = CurrentFrame + 1

        -- The following data layout is: {Frame-Object, New-Size, New-Position}
        local newFrameData = {frameList[CurrentFrame], UDim2.new(0, 405, 0, 370), UDim2.new(1, -650, 1, -500)}		local oldFrameData
        local newBackgroundFrameData
        local oldBackgroundFrameData

        if frameList[CurrentFrame - 1] then oldFrameData = {frameList[CurrentFrame - 1], UDim2.new(0, 320, 0, 292), UDim2.new(1, -950, 1, -450)} oldFrameData.ZIndex = 1 end
        if frameList[CurrentFrame + 1] then newBackgroundFrameData = {frameList[CurrentFrame + 1], UDim2.new(0, 320, 0, 292), UDim2.new(1, -300, 1, -450)} newBackgroundFrameData.ZIndex = 1 end
        if frameList[CurrentFrame - 2] then oldBackgroundFrameData = {frameList[CurrentFrame - 2], UDim2.new(0, 0, 0, 0), UDim2.new(1, -1250, 1, -450)} oldBackgroundFrameData.ZIndex = 1 end
        ChangeFrames({newFrameData, oldFrameData, newBackgroundFrameData, oldBackgroundFrameData})
    end)

    previousButton.MouseButton1Down:Connect(function(x, y)

        if not frameList[CurrentFrame - 1] then return end
        CurrentFrame = CurrentFrame - 1

        -- Same data layout: {Frame-Object, New-Size, New-Position}
        local newFrameData = {frameList[CurrentFrame], UDim2.new(0, 405, 0, 370), UDim2.new(1, -650, 1, -500)}
        local oldFrameData
        local newBackgroundFrameData
        local oldBackgroundFrameData

        if frameList[CurrentFrame + 1] then oldFrameData = {frameList[CurrentFrame + 1], UDim2.new(0, 320, 0, 292), UDim2.new(1, -300, 1, -450)} oldFrameData.ZIndex = 1 end
        if frameList[CurrentFrame - 1] then newBackgroundFrameData = {frameList[CurrentFrame - 1], UDim2.new(0, 320, 0, 292), UDim2.new(1, -950, 1, -450)} newBackgroundFrameData.ZIndex = 1 end
        if frameList[CurrentFrame + 2] then oldBackgroundFrameData = {frameList[CurrentFrame + 2], UDim2.new(0, 0, 0, 0), UDim2.new(1, 0, 1, -450)} oldBackgroundFrameData.ZIndex = 1 end
        ChangeFrames({newFrameData, oldFrameData, newBackgroundFrameData, oldBackgroundFrameData})
    end)

Help?