Model:SetPrimaryPartCFrame() starts to split the model?

I'm using Model:SetPrimaryPartCFrame() to have a model constantly move around by using CFrame values, but after a short amount of time all of the model's parts start to slowly drift apart. Does anybody know why this happens and how to fix it?

i=0
speed=0.0005
model=game.Workspace.Model

while true do
    wait()
    i=i+1
    model.PrimaryPart=model.Primary
    model:SetPrimaryPartCFrame(((planet.CFrame)*(CFrame.fromEulerAnglesXYZ(0, i*speed, 0))*(CFrame.new(0, 0, 100))))
end