Could someone help me fix these values?

I'm having an issue with rotation a model in my script

function rotateMod(Mod,center,rotation)
    local parts ={}
    local function scan(parent)
        for _,obj in pairs(parent:GetChildren()) do
            if (obj:IsA("BasePart")) then
                table.insert(parts,obj)
            end
            scan(obj)
        end
    end
    scan(Mod)
    for _,part in pairs(parts) do
        part.CFrame = (center*rotation*(center:inverse()*part.CFrame))  
    end
end

Mouse = game.Players.LocalPlayer:GetMouse()
Mouse.KeyDown:connect(function(key)
    if selected == true and key == "r" then
        Rot = Rot + 90
        if Rotation == Vector3.new(0,0,0) then Rotation = Vector3.new(0,90,0)
        rotateMod(p,p:GetModelCFrame(),CFrame.new(0,90,0))
        elseif Rotation == Vector3.new(0,90,0) then Rotation = Vector3.new(-180,0,-180)
        rotateMod(p,p:GetModelCFrame(),CFrame.new(-180,0,-180))
        elseif Rotation == Vector3.new(-180,0,-180) then Rotation = Vector3.new(0,-90,0)
        rotateMod(p,p:GetModelCFrame(),CFrame.new(0,-90,0))
        elseif Rotation == Vector3.new(0,-90,0) then Rotation = Vector3.new(0,0,0)
        rotateMod(p,p:GetModelCFrame(),CFrame.new(0,0,0)) end
        s.Rotation = Rotation
    end
end)

I've tried everything I can think of and the rotations never turn correctly. I think its a numerical flaw that I can't figure out. Any help is appreciated.