How can I fix my Vaillant Mag 325 tankless water heater?

I have a Vaillant Mag 325 tankless water heater. For the longest time to get it to fire we have had to run a little water in a sink and then turn on the shower. This gave it adequate flow to fire. In addition the pilot light would go out often. Now we have no hot water. When the water goes through there is a noise from the water heater sounding like cavitation maybe. Does anyone know how to fix this or at least where I can find a manual or parts diagram?