Why isn't a part moving properly between two parts with Heartbeat?

Hi. I'm using Lerp inside a Heartbeat function to move one part from one point to another. This works perfectly when I'm trying to move the part to only one point, but as soon as it starts moving to a second point, it starts stuttering and stops in the middle where it stutters even more back and forth.

This is probably explained much better in a gif, so here's a link

Here's the code

local RunService = game:GetService("RunService")

local partA = workspace.A --point A
local partB = workspace.B --point B
local partC = workspace.C --moving part

local speed = 2

RunService.Heartbeat:Connect(function(deltaTime)
    local deltaSpeed = deltaTime * speed

    local CFrameLerpA = partC.CFrame:Lerp(partA.CFrame, deltaSpeed)
    partC.CFrame = CFrameLerpA
    wait(5)
    local CFrameLerpB = partC.CFrame:Lerp(partB.CFrame, deltaSpeed)
    partC.CFrame = CFrameLerpB
end)