Why is this flickering up and down?

plr = game.Players.LocalPlayer
mouse = plr:GetMouse()
part = Instance.new("Part")
part.Anchored = true
follow = false

mouse.Button1Down:connect(function()
    follow = true
    p = part:Clone()
    p.Parent = workspace
    p.Transparency = .5
    p.CanCollide = false
    p.Anchored = true
    mouse.Button1Up:connect(function()
        follow = false
        p.Transparency = 0
        p.CanCollide = true
    end)
    while follow == true do
        game:GetService('RunService').RenderStepped:wait()
        p.Position = Vector3.new(math.floor(mouse.Hit.p.X),(mouse.Target.Position.Y) + (mouse.Target.Size.Y/2)-(1),math.floor(mouse.Hit.p.Z))
    end
end)

this script it sorta like a building script. everything works except for it flickers up and down and i can't figure out why.

the part is supposed be right above the target like this https://i.gyazo.com/937d78e86f20a1344ba5138a1ce48fd3.png

https://i.gyazo.com/03e8146cf60d336244fb2c3141c9483a.gif

it flickers much faster than the gif shows