Trying to make a part move positions every 100 studs. It only does it once. What did I do wrong?

Hello, menacing_uzi!

You need to add 2 Vector3 so it correctly works, also the variable y is nil in your script

function partNew(color,transparency,position,name)
    local part = Instance.new("Part")
    part.Name = name
    part.BrickColor = color
    part.Transparency = transparency
    part.Reflectance = 0
    part.Anchored = true
    part.CanCollide = true
    part.Position = position
    part.Parent = workspace
    return part
end

local one = partNew(BrickColor.Random(),0.5,Vector3.new(1,1,1),"PartOne")
local randVec = Vector3.new(math.random(100), y, math.random(100))

while true do
    one.Position = one.Position + randVec -- here you were directly setting the position, instead of adding it
    wait(1)
end

EDIT: From what I understood from your question, if you want to teleport the part to a random position inside the 100 studs every second, you need to place the randVec inside the loop

function partNew(color,transparency,position,name)
    local part = Instance.new("Part")
    part.Name = name
    part.BrickColor = color
    part.Transparency = transparency
    part.Reflectance = 0
    part.Anchored = true
    part.CanCollide = true
    part.Position = position
    part.Parent = workspace
    return part
end

local one = partNew(BrickColor.Random(),0.5,Vector3.new(1,1,1),"PartOne")

while true do
    local randVec = Vector3.new(math.random(100), y, math.random(100))
    one.Position = randVec 
    wait(1)
end