Buses Lusaka - Livingstone

Does anyone know how reliable the buses are on the route Lusaka - Livingstone?

I am planning a trip to Europe and I can save a lot of money by relying on the bus, but if I miss it, I will be stuck in Livingstone while my flight takes off from Lusaka...

I am new to the continent and haven't got much public transport experience, except that it all looks a touch chaotic!