NPC Pathfinding Stuttering when a Player is nearby?

Well, it looks like you are not setting the NetworkOwner of the NPC to the server. The problem is when you are far from the NPC, NetworkOwner of your NPC was the server, but when you move close to it, the NetworkOwner changes, and it creates stutter and lag to the NPC.

To fix it, you can simple add a line :

HumanoidRootPart:SetNetworkOwner(nil) -- 'nil' equals server

And your code might look like this :

local PathfindingService = game:GetService("PathfindingService")
local Nightmare = script.Parent

local Humanoid = Nightmare:WaitForChild("Humanoid")
local HumanoidRootPart = Nightmare:FindFirstChild("HumanoidRootPart")

--##[POSSIBLE POSITIONS]##--

local Pos1 = Vector3.new(-77.34, -11.133, 7.23)
local Pos2 = Vector3.new(-9.93, 4.777, 7.23)
local Pos3 = Vector3.new(43.15, -9.373, 19.86)
local Pos4 = Vector3.new(13.49, -6.183, 39.12)
local Pos5 = Vector3.new(-2.97, -6.183, 77.37)
local Pos6 = Vector3.new(-61.12, -6.183, 46.7)
local Pos7 = Vector3.new(-31.46, -6.183, -1.03)
local Pos8 = Vector3.new(-24.45, 8.907, -20.1)
local Pos9 = Vector3.new(4.3, 8.907, -24.05)
local Pos10 = Vector3.new(3.71, 8.907, -4.79)

--##[MOVE]##--

while true do
HumanoidRootPart:SetNetworkOwner(nil)
local Randomizer = math.random(1, 10)

    local Path = PathfindingService:CreatePath()
        if Randomizer == 1 then
        Path:ComputeAsync(HumanoidRootPart.Position, Pos1)
    end

        if Randomizer == 2 then
        Path:ComputeAsync(HumanoidRootPart.Position, Pos2)
    end

        if Randomizer == 3 then
        Path:ComputeAsync(HumanoidRootPart.Position, Pos3)
    end

    if Randomizer == 4 then
        Path:ComputeAsync(HumanoidRootPart.Position, Pos4)
    end

    if Randomizer == 5 then
        Path:ComputeAsync(HumanoidRootPart.Position, Pos5)
    end

    if Randomizer == 6 then
        Path:ComputeAsync(HumanoidRootPart.Position, Pos6)
    end

    if Randomizer == 7 then
        Path:ComputeAsync(HumanoidRootPart.Position, Pos7)
    end

    if Randomizer == 8 then
        Path:ComputeAsync(HumanoidRootPart.Position, Pos8)
    end

    if Randomizer == 9 then
        Path:ComputeAsync(HumanoidRootPart.Position, Pos9)
    end

    if Randomizer == 10 then
        Path:ComputeAsync(HumanoidRootPart.Position, Pos10)
    end


    if Path.Status == Enum.PathStatus.Success then
        local Waypoints = Path:GetWaypoints()
        for _, Waypoint in pairs(Waypoints) do
            Humanoid:MoveTo(Waypoint.Position)

            if Waypoint.Action == Enum.PathWaypointAction.Jump then
                Humanoid:ChangeState(Enum.HumanoidStateType.Jumping)
                end

            Humanoid.MoveToFinished:Wait()
        end
    end
    wait(5)
end

Lemme know if it helps!